[DAGCombiner][SVE] Ensure MGATHER/MSCATTER addressing mode combines preserve index scaling

refineUniformBase and selectGatherScatterAddrMode both attempt the
transformation:

  base(0) + index(A+splat(B)) => base(B) + index(A)

However, this is only safe when index is not implicitly scaled.

Differential Revision: https://reviews.llvm.org/D123222