[llvm] [SimplifyCFG] Simplify conditional branches on const icmp eq's (PR #73334)

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yonillasky wrote:

> What is the broader motivation for this? That is, why is it important to handle this within a single SimplifyCFG invocation? We'd probably want to have a PhaseOrdering test for that.

Pay close attention to the test. To fully simplify the test case with existing code you'd need to run SimplifyCFG first, then a pass that folds constants, then SimplifyCFG again.
That, to me, seemed unreasonable - hence the PR.

> I'm not entirely sure whether this is the best approach. A possible alternative would be to instead perform additional simplification when removing the unreachable block and simplifying the phi to a constant. We could try to simplify all instructions that used the phi at that point.

I like the idea. However, how about specifically attempting to constant-fold the successor's BranchInst condition, if it has one, then?
I'm making this counter-offer since I'm not sure the CFG simplification pass should be also doing constant-folding in unrelated instructions.

https://github.com/llvm/llvm-project/pull/73334