[AArch64] Fix i64 nontemporal high-half extraction.

Since we only have pair - not single - nontemporal store instructions,
we have to extract the high part into a separate register to be able
to use them.

When the initial nontemporal codegen support was added, I wrote the
extract using the nonsensical UBFX [0,32[.
Use the correct LSR form instead.

llvm-svn: 259134