[ARM][MVE] Optimise offset addresses of gathers/scatters

This patch adds an analysis of the offset addresses used by gathers
and scatters to the MVEGatherScatterLowering pass to find
multiplications and additions that are loop invariant and thus can
be moved into the loop preheader, avoiding to execute them each time.

Differential Revision: https://reviews.llvm.org/D76681