[llvm] [InstCombine] Enable FAdd simplifications when user can ignore sign bit (PR #157757)

[Vedant Paranjape via llvm-commits]

VedantParanjape wrote:

@arsenm tracking here for which call operators this can be implemented.

- [ ] visitFNeg
- [X] visitFAdd
- [ ] visitFSub
- [ ] visitFMul
- [ ] visitFRem
- [ ] visitFDiv

https://github.com/llvm/llvm-project/pull/157757