[AArch64][SVE] Add more folds to make use of gather/scatter with 32-bit indices

In AArch64ISelLowering.cpp this patch implements this fold:

1) GEP (%ptr, SHL ((stepvector(A) + splat(%offset))) << splat(B)))
into GEP (%ptr + (%offset << B), step_vector (A << B))

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 D117900

Differential Revision: https://reviews.llvm.org/D118345