[ARM,MVE] Add intrinsics for vector comparisons.

This adds the `vcmp` family of ACLE MVE intrinsics: vector/vector,
vector/scalar, and the predicated forms of both. All are represented
using standard existing IR: vector/scalar comparisons are represented
by making a vector out of the scalar first, and predicated forms are
represented by taking the bitwise AND of the input predicate and the
output of the comparison. Existing LLVM-side tests demonstrate that
ISel will pattern-match all of that back down to single MVE VCMPs.

The idiom of handling a vector/scalar operation by generating IR to
expand the scalar into a second vector is going to be needed for a lot
of MVE intrinsics, so to make that easy, I've provided a helper
function that automatically works out the element count.

The comparison intrinsics are the first ones that have to //return// a
predicate, in the user-facing `mve_pred16_t` format. This means we
have to use the `arm_mve_pred_v2i` low-level intrinsic to convert it
back from the logical `<n x i1>` form used in IR. I've done that
explicitly in the code gen specification for the builtins, because it
happens much more rarely in the ACLE API than passing a Predicate as
input, so it didn't seem worth automating in MveEmitter.

Reviewers: ostannard, MarkMurrayARM, dmgreen

Reviewed By: dmgreen

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D70297