[ARM][MVE] Teach MVEGatherScatterLowering to merge successive getelementpointers

A patch following up on the introduction of pointer induction variables, adding
a preprocessing step to the address optimisation in the MVEGatherScatterLowering
pass. If the getelementpointer that is the address is itself using a
getelementpointer as base, they will be merged into one by summing up the
offsets, after checking that this will not cause an overflow (this can be
repeated recursively).

Differential Revision: https://reviews.llvm.org/D84027