AArch64: use 32-bit MOV rather than UBFX to truncate registers.

It's potentially more efficient on Cyclone, and from the optimization guides &
schedulers looks like it has no effect on Cortex-A53 or A57. In general you'd
expect a MOV to be about the most efficient instruction with its semantics,
even though the official "UXTW" alias is really a UBFX.

llvm-svn: 243576