Refactor CondBr: remove abstraction or actually pass in registers to compare

[mooori]

According to my understanding, CondBr currently receives an IntegerCompare as kind. However, the registers rs1 and rs2 of this IntegerCompare don’t contain values to compare. Instead rs1 contains the result of the comparison related to CondBr and rs2 is set to zero_reg. This is based on:

• ISLE rules
• lowering functions (in particular this line and how test is passed subsequently into IntegerCompare)

Potential refactorings

1) Change CondBr structure

Brought up by @nagisa in this comment:

Change CondBr to something like CondBr { condition: ..., target: ..., rs1: ... } which would no longer imply that it compares stuff internally.

2) Compare stuff in CondBr

To avoid misinterpretations of the kind field of CondBr. I assume in this case rs1 and rs2 should be the registers to be compared.

Notes

@Akashin pointed out that later on it might be necessary to use CondBr in ways incompatible with the refactoring described in 1). Such use cases can be found in emit.rs, though they are currently commented out. More generally, it might be too early to refactor CondBr now.