[ARM,MVE] Add vector-scalar intrinsics

Summary:
This patch adds vector-scalar variants to the following families of
MVE intrinsics:
* vaddq
* vsubq
* vmulq
* vqaddq
* vqsubq
* vhaddq
* vhsubq
* vqdmulhq
* vqrdmulhq

The vector-scalar variants perform a splat operation on the scalar
operand and then perform the same operations as their vector-vector
counterparts. Code generation is done accordingly (using LLVM IR 'insert'
and 'shuffle' operations which are later converted into an ARMvdup
SDNode).

Reviewers: simon_tatham, dmgreen, MarkMurrayARM, ostannard

Reviewed By: dmgreen

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74620