Do not build LLVM bitcast instructions for opaque pointer casts

[HertzDevil]

Part of #12743. When opaque pointers are available, casts between pointers (in the same address space) are always no-ops in LLVM, but this alone doesn't prevent Crystal from computing the pointee type. This PR does that by not evaluating the type argument at all for LLVM 15+. Note that all uses of bitcast in Crystal are for pointer casts.

[HertzDevil]

Moment of truth:

• LLVM 14.0.1, c6e3116 (one commit before Support typed LLVM getelementptr instructions #12623)

$ git checkout c6e3116df9a44a871740318ebcf486f986b79dad
$ make clean crystal clean_cache release=1 && touch src/compiler/crystal.cr && make stats=1
Parse:                             00:00:00.000214084 (   0.77MB)
Semantic (top level):              00:00:00.345461000 ( 173.16MB)
Semantic (new):                    00:00:00.001670375 ( 173.16MB)
Semantic (type declarations):      00:00:00.030641833 ( 173.16MB)
Semantic (abstract def check):     00:00:00.012177750 ( 189.16MB)
Semantic (restrictions augmenter): 00:00:00.011101791 ( 205.16MB)
Semantic (ivars initializers):     00:00:04.578064625 (1813.06MB)
Semantic (cvars initializers):     00:00:00.004923458 (1813.06MB)
Semantic (main):                   00:00:05.646283959 (2197.06MB)
Semantic (cleanup):                00:00:00.000617875 (2197.06MB)
Semantic (recursive struct check): 00:00:00.001288625 (2197.06MB)
Codegen (crystal):                 00:00:03.563826917 (2669.06MB)
Codegen (bc+obj):                  00:00:06.565988417 (2669.06MB)
Codegen (linking):                 00:00:00.878433167 (2669.06MB)
dsymutil:                          00:00:00.558394875 (2669.06MB)

Macro runs:
 - /Users/quinton/crystal/crystal/src/ecr/process.cr: 00:00:04.653824125

Codegen (bc+obj):
 - no previous .o files were reused

• LLVM 14.0.1, this PR

$ git checkout cfd9d60de08a0c5988816402f7722fb44355bbb6
$ make clean crystal clean_cache release=1 && touch src/compiler/crystal.cr && make stats=1
Parse:                             00:00:00.000189375 (   0.77MB)
Semantic (top level):              00:00:00.341806000 ( 172.08MB)
Semantic (new):                    00:00:00.001625917 ( 172.08MB)
Semantic (type declarations):      00:00:00.029480709 ( 172.08MB)
Semantic (abstract def check):     00:00:00.012977834 ( 204.08MB)
Semantic (restrictions augmenter): 00:00:00.009416000 ( 204.08MB)
Semantic (ivars initializers):     00:00:04.530023875 (1828.00MB)
Semantic (cvars initializers):     00:00:00.005073917 (1828.00MB)
Semantic (main):                   00:00:06.559616208 (2140.00MB)
Semantic (cleanup):                00:00:00.000502583 (2140.00MB)
Semantic (recursive struct check): 00:00:00.001180125 (2140.00MB)
Codegen (crystal):                 00:00:02.610722417 (2732.02MB)
Codegen (bc+obj):                  00:00:06.334841000 (2732.02MB)
Codegen (linking):                 00:00:01.092296084 (2732.02MB)
dsymutil:                          00:00:00.696052500 (2732.02MB)

Macro runs:
 - /Users/quinton/crystal/crystal/src/ecr/process.cr: 00:00:04.650499750

Codegen (bc+obj):
 - no previous .o files were reused

• LLVM 15.0.7, this PR

$ git checkout cfd9d60de08a0c5988816402f7722fb44355bbb6
$ make clean crystal clean_cache release=1 && touch src/compiler/crystal.cr && make stats=1
Parse:                             00:00:00.000174458 (   0.77MB)
Semantic (top level):              00:00:00.419430167 ( 173.17MB)
Semantic (new):                    00:00:00.001808792 ( 173.17MB)
Semantic (type declarations):      00:00:00.030975167 ( 173.17MB)
Semantic (abstract def check):     00:00:00.012816666 ( 189.17MB)
Semantic (restrictions augmenter): 00:00:00.010163750 ( 205.17MB)
Semantic (ivars initializers):     00:00:06.030664375 (1813.08MB)
Semantic (cvars initializers):     00:00:00.004775667 (1813.08MB)
Semantic (main):                   00:00:06.327635500 (2205.08MB)
Semantic (cleanup):                00:00:00.000621167 (2205.08MB)
Semantic (recursive struct check): 00:00:00.001355583 (2205.08MB)
Codegen (crystal):                 00:00:02.735493083 (2685.09MB)
Codegen (bc+obj):                  00:00:05.499450291 (2685.09MB)
Codegen (linking):                 00:00:00.426940250 (2685.09MB)
dsymutil:                          00:00:00.593159500 (2685.09MB)

Macro runs:
 - /Users/quinton/crystal/crystal/src/ecr/process.cr: 00:00:03.145058791

Codegen (bc+obj):
 - no previous .o files were reused

Maximum memory load is not as bad as I thought, despite the addition of Crystal::CodeGenVisitor#@fun_types.

Codegen (crystal) actually takes less time now, regardless of whether LLVM supports opaque pointers. Improvements in Codegen (bc+obj) are solely due to LLVM and beyond Crystal's direct control; they are not very conclusive as I am not building the compiler in release mode for the second generation. On the other hand, quite a lot more time is spent on the semantic phase, which I can't quite relate to the LLVM changes (yet the times for macro runs have reduced by almost the same duration).

I will be doing a few more runs of make clean crystal clean_cache release=1 && bin/crystal run --stats src/compiler/crystal.cr to see if there is imminent need for further optimizations, i.e. precomputing call types where they are known in advance instead of caching them.

[HertzDevil]

Here are the stats of make clean crystal release=1, followed by 10 runs of make clean_cache && bin/crystal run --stats src/compiler/crystal.cr:

	LLVM 14.0.1 (c6e3116)	LLVM 14.0.1 (cfd9d60)	LLVM 15.0.7 (cfd9d60)
Parse	0.0000s (±21.8%)	0.0000s (±17.3%)	0.0000s (±13.9%)
Semantic (top level)	0.4296s (±3.6%)	0.4528s (±9.0%)	0.4658s (±4.7%)
Semantic (new)	0.0018s (±2.6%)	0.0018s (±12.7%)	0.0019s (±17.5%)
Semantic (type declarations)	0.0308s (±3.2%)	0.0310s (±4.6%)	0.0329s (±16.9%)
Semantic (abstract def check)	0.0127s (±1.7%)	0.0131s (±3.0%)	0.0129s (±2.3%)
Semantic (restrictions augmenter)	0.0101s (±4.4%)	0.0101s (±8.9%)	0.0098s (±6.9%)
Semantic (ivars initializers)	4.5505s (±0.8%)	4.5354s (±0.7%)	6.0346s (±2.6%)
Semantic (cvars initializers)	0.0051s (±3.4%)	0.0053s (±9.4%)	0.0050s (±2.0%)
Semantic (main)	6.3904s (±1.9%)	6.3909s (±2.8%)	5.5233s (±3.2%)
Semantic (cleanup)	0.0006s (±4.2%)	0.0006s (±2.4%)	0.0006s (±2.7%)
Semantic (recursive struct check)	0.0012s (±3.6%)	0.0012s (±2.8%)	0.0012s (±8.6%)
Codegen (crystal)	2.6454s (±9.7%)	2.7394s (±3.5%)	2.8363s (±3.8%)
Codegen (bc+obj)	5.9099s (±2.8%)	6.0044s (±2.3%)	5.1995s (±0.8%)
Codegen (linking)	0.7582s (±1.5%)	0.7576s (±1.4%)	0.3392s (±19.1%)
dsymutil	0.3473s (±2.0%)	0.3486s (±1.6%)	0.3940s (±2.9%)
Macro runs	4.6345s (±1.4%)	4.6609s (±1.9%)	3.0254s (±3.3%)

Plus 1 run in release mode (only the stages beyond Crystal's control are shown because the other times are unaffected):

	LLVM 14.0.1 (c6e3116)	LLVM 14.0.1 (cfd9d60)	LLVM 15.0.7 (cfd9d60)
Codegen (bc+obj)	247.8567s	251.4313s	184.3743
Codegen (linking)	0.4536s	0.4714s	0.3295s
dsymutil	0.6281s	0.6342s	0.5236s