[AArch64][SVE] Add support for trunc to <vscale x N x i1>.

This isn't a natively supported operation, so convert it to a
mask+compare.

In addition to the operation itself, fix up some surrounding stuff to
make the testcase work: we need concat_vectors on i1 vectors, we need
legalization of i1 vector truncates, and we need to fix up all the
relevant uses of getVectorNumElements().

Differential Revision: https://reviews.llvm.org/D83811