[ARM,MVE] Add ACLE intrinsics for VQMOV[U]N family.

Summary:
These instructions work like VMOVN (narrowing a vector of wide values
to half size, and overwriting every other lane of an output register
with the result), except that the narrowing conversion is saturating.
They come in three signedness flavours: signed to signed, unsigned to
unsigned, and signed to unsigned. All are represented in IR by a
target-specific intrinsic that takes two separate 'unsigned' flags.

Reviewers: MarkMurrayARM, dmgreen, miyuki, ostannard

Reviewed By: dmgreen

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D75252