[SVE] Update EmitSVEPredicateCast to take a ScalableVectorType

Summary:
Removes usage of VectorType::getNumElements identified by test located
at CodeGen/aarch64-sve-intrinsics/acle_sve_abs.c. Since the type is an
SVE predicate vector, it makes sense to specialize the code for scalable
vectors only.

Reviewers: rengolin, efriedma

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, rkruppe, psnobl, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78958