[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