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.
Is this the same as or similar enough to CreateVectorSplat to use that instead?