optimizer fixes and guardrails

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

chore: refactor and improve optimizer

this commit factors out and generalizes the optimizer logic for
comparison operators. it clarifies the logic for dealing with boundary
cases, and cleans up the control flow in the optimizer in general. it
also adds an assembly peephole optimization to help optimize the
assembly generated by the comparator pass.

this commit also introduces a new IR keyword, "unique_symbol". it
functions as a guardrail to help ensure optimizer passes are sane. it is
a statement that codegen can insert into the IR, and sanity checks will
be performed to make sure that the statement is not optimized out, and
in some cases, to ensure that the statement shows up in the optimized
IR. it can be thought of as a very primitive effects tracking framework.

lastly, some tests are added to increase optimizer coverage. 

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #2914 (4d9e8bb) into master (4e38ec4) will decrease coverage by 0.42%.
The diff coverage is 95.37%.

@@            Coverage Diff             @@
##           master    #2914      +/-   ##
==========================================
- Coverage   88.47%   88.05%   -0.43%     
==========================================
  Files          97       97              
  Lines       10613    10899     +286     
  Branches     2527     2637     +110     
==========================================
+ Hits         9390     9597     +207     
- Misses        770      836      +66     
- Partials      453      466      +13     

Impacted Files	Coverage Δ
vyper/ir/compile_ir.py	93.43% <72.72%> (-0.26%)	⬇️
vyper/codegen/core.py	84.93% <83.33%> (+0.34%)	⬆️
vyper/ir/optimizer.py	95.01% <96.49%> (+2.78%)	⬆️
vyper/builtin_functions/functions.py	92.40% <100.00%> (-1.73%)	⬇️
vyper/codegen/external_call.py	97.34% <100.00%> (+0.02%)	⬆️
vyper/codegen/ir_node.py	92.28% <100.00%> (+1.23%)	⬆️
vyper/codegen/self_call.py	95.65% <100.00%> (+0.19%)	⬆️
vyper/compiler/__init__.py	88.09% <100.00%> (+0.91%)	⬆️
vyper/compiler/output.py	93.78% <100.00%> (ø)
vyper/codegen/function_definitions/utils.py	80.00% <0.00%> (-20.00%)	⬇️
... and 12 more

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[lgtm-com]

This pull request introduces 1 alert when merging c148926 into 4e38ec4 - view on LGTM.com

new alerts:

• 1 for Unused local variable

[lgtm-com]

This pull request introduces 1 alert when merging c057cb6 into 4e38ec4 - view on LGTM.com

new alerts:

• 1 for Unused local variable