[ARM,MVE] Add intrinsics for the VQDMLAD family.

Summary:
This is another set of instructions too complicated to be sensibly
expressed in IR by anything short of a target-specific intrinsic.
Given input vectors a,b, the instruction generates intermediate values
2*(a[0]*b[0]+a[1]+b[1]), 2*(a[2]*b[2]+a[3]+b[3]), etc; takes the high
half of each double-width values, and overwrites half the lanes in the
output vector c, which you therefore have to provide the input value
of. Optionally you can swap the elements of b so that the are things
like a[0]*b[1]+a[1]*b[0]; optionally you can round to nearest when
taking the high half; and optionally you can take the difference
rather than sum of the two products. Finally, saturation is applied
when converting back to a single-width vector lane.

Reviewers: dmgreen, MarkMurrayARM, miyuki, ostannard

Reviewed By: miyuki

Subscribers: kristof.beyls, hiraditya, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76359