[AArch64][SVE] Fold gather/scatter with 32bits when possible

In AArch64ISelLowering.cpp this patch implements this fold:

GEP (%ptr, (splat(%offset) + stepvector(A)))
into GEP ((%ptr + %offset), stepvector(A))

The above transform simplifies the index operand so that it can be expressed
as i32 elements.
This allows using only one gather/scatter assembly instruction instead of two.

Patch by Paul Walker (@paulwalker-arm).

Depends on D118459

Differential Revision: https://reviews.llvm.org/D117900