[InstCombine] fold fcmp-of-copysign idiom

As discussed in:
https://llvm.org/PR49179
...this pattern shows up in library code.
There are several potential generalizations as noted,
but we need to be careful that we get FP special-values
right, and it's not clear how much variation we should
expect to see from this exact idiom.

GitOrigin-RevId: 85294703a74a532448b3d3801d4e9a5c6024547a
2 files changed