[llvm] [PowerPC] vector shift word/double by element size - 1 use all ones (PR #139794)

[Maryam Moghadas via llvm-commits]

https://github.com/maryammo approved this pull request.

Could you please elaborate on the comment at line 18503 in PPCISelLowering.cpp  to clarify that all-ones vector as the shift amount is valid because PPC vector shift instructions (vslw /vsld / vsraw / vsrad ) only utilize the lower 5 or 6 bits of each element.

https://github.com/llvm/llvm-project/pull/139794