[AArch64][SVE] Zero other lanes when doing OR reduction on unpacked predicate using ptest.

When the predicate vector is unpacked, we cannot assume anything about the
values in the other lanes. We have to make sure we use the correct
predicate where we know that the other lanes have been zeroed.

Reviewed By: RosieSumpter

Differential Revision: https://reviews.llvm.org/D129081