[llvm] [AMDGPU] Omit umin on ctlz/cttz if operand is non-zero. (PR #79127)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Instead of doing this during the lowering, should the combine on CTLZ/CTTZ transform the non-undef version into the undef version if the input is known non-zero? I thought it was already doing that 

https://github.com/llvm/llvm-project/pull/79127