[ARM] Avoid pointless vrev of element-wise vmov

If we have an element-wise vmov immediate instruction then a subsequent vrev
with width greater or equal to the vmov element width, then that vrev won't do
anything. Add a DAG combine to convert bitcasts that would become such vrevs
into vector_reg_casts instead.

Differential Revision: https://reviews.llvm.org/D76514