[llvm] [AMDGPU] Fold isnan+fptosi select to fptosi (PR #200960)

[Krzysztof Drewniak via llvm-commits]

https://github.com/krzysz00 commented:

Logic seems correct, though I'll leave it to others to have thoughts on if these should be autogenerated (perhaps with some `--implicit-check-not`) or if this is fine.

https://github.com/llvm/llvm-project/pull/200960