[ARM][MVE][Intrinsics] Add VMULH/VRMULH intrinsics.

Summary: Add MVE VMULH/VRMULH intrinsics and unit tests.

Reviewers: simon_tatham, ostannard, dmgreen

Subscribers: kristof.beyls, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D70948