[llvm] [LLVM] Make use of s_flbit_i32_b64 and s_ff1_i32_b64 (PR #75158)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

LGTM with test tweaks.

Should also do GlobalISel as Matt mentioned, but that can be a separate patch.

https://github.com/llvm/llvm-project/pull/75158