[SVE][Codegen] Add a helper function for pointer increment logic

Summary:
Helper used when splitting load & store operations to calculate
the pointer + offset for the high half of the split

Reviewers: efriedma, sdesmalen, david-arm

Reviewed By: efriedma

Subscribers: tschuett, hiraditya, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83577