[clang][AArch64][SVE] Avoid going through memory for fixed/scalable predicate casts

For fixed SVE types, predicates are represented using vectors of i8,
where as for scalable types they are represented using vectors of i1. We
can avoid going through memory for casts between these by bitcasting the
i1 scalable vectors to/from a scalable i8 vector of matching size, which
can then use the existing vector insert/extract logic.

Differential Revision: https://reviews.llvm.org/D106860