[AArch64] NFC: Move safe predicate casting to a separate function.

This patch puts the code to safely bitcast a predicate, and possibly zero
any undefined lanes when doing a widening cast, into one place and merges
the functionality with lowerConvertToSVBool.

This is some cleanup inspired by D128665.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D128926