[llvm] [GlobalIsel] Canonicalize G_FCMP (PR #108891)

[Amara Emerson via llvm-commits]

https://github.com/aemerson commented:

Github won't let me inline comment on code that wasn't modified, but in `llvm::GIConstant::getConstant()` where you call `getIConstantVRegValWithLookThrough()`: it just occurred to me that we could end up with the width of the APInt being different from the width of the original `Register Const` if the look through went through some SEXT/ZEXT operations. Do you agree?

https://github.com/llvm/llvm-project/pull/108891