[SelectionDAGBuilder] Don't create MGATHER/MSCATTER with Scale != ElemSize

This is an alternative to D124530. In getUniformBase() only create
scales that match the gather/scatter element size. If targets also
support other scales, then they can produce those scales in target
DAG combines. This is what X86 already does (as long as the
resulting scale would be 1, 2, 4 or 8).

This essentially restores the pre-opaque-pointer state of things.

Fixes https://github.com/llvm/llvm-project/issues/55021.

Differential Revision: https://reviews.llvm.org/D124605