[AArch64][SVE] Improve code generation for VLS i1 masks

This patch partially resolves an issue for VLS code generation
where a mask is generated from a smaller width integer comparison
than the instruction using the mask requires.

Instead of sign extending a p register by converting it to a z
register, extending that, and converting back, we instead just
do an unpack of the p register.

A separate issue causes the code generation to still be poor when
the mask generation would fit in a neon register, as we then use
a neon comparison operation and have to convert that to a p register.
This will be resolved in a separate patch.

Reviewed By: peterwaller-arm

Differential Revision: https://reviews.llvm.org/D111221