ARM64: add correct vector registers during asm parsing

Previously, we ignored the difference between V64 and V128 when parsing
assembly: they both got mapped to registers in the FPR128 class. This is
basically harmless at the moment because they both print and encode the same
way. However, it will affect the printing of aliases.
TableGen/ARM64: print aliases even if they have syntax variants.

To get at least one use of the change (and some actual tests) in with its
commit, I've enabled the AArch64 & ARM64 NEON mov aliases.