[SVE] Add support for non-element-type sized scaling when lowering MGATHER/MSCATTER.

[SVE] Add support for non-element-type sized scaling when lowering MGATHER/MSCATTER.

The lowering code did not use the scale operand of MGATHER/MSCATTER
nodes, but instead assumed scaled indices were always scaled based
on the element type of the memory type. This patch adds the missing
support by rewritting the nodes as unscaled variants.

Differential Revision: https://reviews.llvm.org/D123670