[ARM] Widening loads and narrowing stores

[ARM] Widening loads and narrowing stores

MVE has instructions to widen as it loads, and narrow as it stores. This adds
the required patterns and legalisation to make them work including specifying
that they are legal, patterns to select them and test changes.

Patch by David Sherwood.

Differential Revision: https://reviews.llvm.org/D63839

llvm-svn: 364636