[LegalizeDAG][X86][AMDGPU] Use ANY_EXTEND instead of ZERO_EXTEND when...

[LegalizeDAG][X86][AMDGPU] Use ANY_EXTEND instead of ZERO_EXTEND when promoting ISD::CTTZ/CTTZ_ZERO_UNDEF.

Summary:
For CTTZ we place a set bit just past where the non-promoted type
stopped so the extended bits won't be used for the count. For
CTTZ_ZERO_UNDEF we don't care what happens if no bits are set in
the original type and we end up counting into the extended bits.
So we can just use ANY_EXTEND for both cases.

This matches what is done in type legalization for these operations.
We make no effort to force the upper bits to zero.

Differential Revision: https://reviews.llvm.org/D74111