[ARM] Add a batch of MVE integer instructions.

This includes integer arithmetic of various kinds (add/sub/multiply,
saturating and not), and the immediate forms of VMOV and VMVN that
load an immediate into all lanes of a vector.

Reviewers: dmgreen, samparker, SjoerdMeijer, t.p.northover

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62674

llvm-svn: 363936