[ARM][MVE] VPT Blocks: findVCMPToFoldIntoVPS

This is a recommit of D71330, but with a few things fixed and changed:

1) ReachingDefAnalysis: this was not running with optnone as it was checking
skipFunction(), which other analysis passes don't do. I guess this is a
copy-paste from a codegen pass.
2) VPTBlockPass: here I've added skipFunction(), because like most/all
optimisations, we don't want to run this with optnone.

This fixes the issues with the initial/previous commit: the VPTBlockPass was
running with optnone, but ReachingDefAnalysis wasn't, and so VPTBlockPass was
crashing querying ReachingDefAnalysis.

I've added test case mve-vpt-block-optnone.mir to check that we don't run
VPTBlock with optnone.

Differential Revision: https://reviews.llvm.org/D71470