[llvm] [AMDGPU] Clean up `LowerFP_TO_INT_SAT` (NFC) (PR #206118)

[Igor Wodiany via llvm-commits]

IgWod wrote:

@jayfoad Let me know if that's what you had in mind and whether there is anything else you think that needs improving in this function.

https://github.com/llvm/llvm-project/pull/206118