[ARM] Push gather/scatter shl index updates out of loops

This teaches the MVE gather scatter lowering pass that SHL is
essentially the same as Mul, where we are able to optimize the
induction of a gather/scatter address by pushing them out of loops.
https://alive2.llvm.org/ce/z/wG4VyT

Differential Revision: https://reviews.llvm.org/D112920