[AMDGPU] Use LSH for lowering ctlz_zero_undef.i8/i16 (#88512)

Use LSH to lower ctlz_zero_undef instead of subtracting leading zeros
for i8 and i16.

Related to [77615](https://github.com/llvm/llvm-project/pull/77615

).

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Co-authored-by: Leon Clark <leoclark@amd.com>