[ARM] Add MVE vector compare instructions.
These take a pair of vector register to compare, and a comparison type
(written in the form of an Arm condition suffix); they output a vector
of booleans in the VPR register, where predication can conveniently
Reviewers: dmgreen, samparker, SjoerdMeijer, t.p.northover
Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D62676