[ARM] Always replace FP16 bitcasts with VMOVhr or VMOVrh

This changes the logic with lowering fp16 bitcasts to always produce
either a VMOVhr or a VMOVrh, instead of only trying to do it with
certain surrounding nodes. To perform the same optimisations demand bits
and known bits information has been added for them.

Differential Revision: https://reviews.llvm.org/D78587