[AArch64][SVE] Implement extractelement of i1 vectors.

The implementation just extends the vector to a larger element type, and
extracts from that.  Not fancy, but generates reasonable code.

There was discussion in the review of doing the promotion in
target-independent code, but I'm sticking with this to avoid making
LegalizeDAG infrastructure more complicated.

Differential Revision: https://reviews.llvm.org/D87651