AArch64/ARM64: emit all vector FP comparisons as such.

ARM64 was scalarizing some vector comparisons which don't quite map to
AArch64's compare and mask instructions. AArch64's approach of sacrificing a
little efficiency to emulate them with the limited set available was better, so
I ported it across.

More "inspired by" than copy/paste since the backend's internal expectations
were a bit different, but the tests were invaluable.

llvm-svn: 206570