[llvm] [RISCV] Canonicalize foldable branch conditions in optimizeCondBranch (PR #132988)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland approved this pull request.

This patch will be helpful for dynamic instruction count since we avoid the load. For that reason, I am inclined to accept this patch. However, there are two related issues that are still left:
1. We may miss the opportunity to canonicalize if the `li + condbr` pair is introduced later on in the pipeline. I am not aware of whether this occurs or not.
2. We miss the opportunity to turn these branches into an unconditional jump. I wonder if this kind of optimization would be beneficial for branch prediction. Maybe the a LatePeephole pass could just look to modify the CFG if it sees this canonical case.

If (1) does not occur, then I think it is appropriate to proceed forward with accepting this patch, followed up with the suggestion in (2), and there is no need to evaluate the conditional branch statically during a late peephole pass. However, if we are introducing these pairs later in the pipeline, I still think this patch should be landed since we may benefit from optimizing earlier and then the late peephole pass should both evaluate and simplify.

WDYT?

https://github.com/llvm/llvm-project/pull/132988