[clang] [CIR][AMDGPU] Use nsw add and ordered compare in logb lowering (PR #228004)

[Steffen Larsen via cfe-commits]

steffenlarsen wrote:

@aobolensk - I think you merged this after https://github.com/llvm/llvm-project/pull/222018, but forgot to address the FIXME in it.

https://github.com/llvm/llvm-project/pull/228004