AArch64: fix big-endian immediate materialisation

We were materialising big-endian constants using DAG nodes with types different
from what was requested, followed by a bitcast. This is fine on little-endian
machines where bitcasting is a nop, but we need a slightly different
representation for big-endian. This adds a new set of NVCAST (natural-vector
cast) operations which are always nops.

Patch by Asiri Rathnayake.