[ARM] Better OR's for MVE compares

This adds a DeMorgan combine for OR's of compares to turn them into AND's,
helping prevent them from going into and out of gpr registers. It also fills in
the VCLE and VCLT nodes that MVE can select, allowing it to invert more
compares.

Differential Revision: https://reviews.llvm.org/D65059

llvm-svn: 366920