[llvm] [AMDGPU] Fix OpenCL conformance test failures for ctlz. (PR #83170)

[Jay Foad via llvm-commits]

jayfoad wrote:

The version with the shift works fine for ctlz_zero_undef (and it would still be good to generate it since it's usually shorter) but not ctlz.

https://github.com/llvm/llvm-project/pull/83170