[VE] v256.32|64 gather|scatter isel and tests

This adds support for v256.32|64 scatter|gather isel.  vp.gather|scatter
and regular gather|scatter intrinsics are both lowered to the internal
VVP layer.  Splitting these ops on v512.32 is the subject of future
patches.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D121288