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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > 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 (it is https://github.com/llvm/llvm-project/blob/9731b77e80261c627d79980f8c275700bdaf6591/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L11005C6-L11005C7)
> 
> It must be missing some cases. Otherwise the tests wouldn't change.

Yes, so should debug why that happened. We shouldn't need to reinvent optimizations during the lowering 

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