[AArch64][SVE] Implement SPLAT_VECTOR for i1 vectors.

The generated sequence with whilelo is unintuitive, but it's the best
I could come up with given the limited number of SVE instructions that
interact with scalar registers. The other sequence I was considering
was something like dup+cmpne, but an extra scalar instruction seems
better than an extra vector instruction.

Differential Revision: https://reviews.llvm.org/D71160