[llvm] [InstCombine] Fold `(X==Z || Y==Z) ? (X==Z && Y==Z) : X==Y --> X==Y` (PR #108619)

[Marina Taylor via llvm-commits]

citymarina wrote:

> > > It looks like InstCombine canonicalizes this to something like `(X == Z) ? (Y == Z) : !(Y == Z) && X == Y)`, see https://alive2.llvm.org/ce/z/XY_HMX. Shouldn't we be matching that pattern instead?
> > 
> > 
> > I am new in this area and unfamiliar with the conventions. My thought process was that matching something closer to the Swift-generated IR would better express the intent of this patch, and also I wasn't sure if I could rely on the altered form remaining stable over time. But if you think it would be better then I'm happy to update the patch.
> 
> Typically canonical patterns are what remain stable longer. They aren't folds that meaningful change the instructions, just standardize them in a way so that future matching code only needs to worry about a single iteration. You can see more about it: https://llvm.org/docs/InstCombineContributorGuide.html#canonicalization-and-target-independence

That makes sense. I guess the missing piece for me was that I didn't realize `(X == Z) ? (Y == Z) : !(Y == Z) && X == Y)` was in fact a canonicalization. I'll work on an update.

https://github.com/llvm/llvm-project/pull/108619