AArch64: simplify tbl/tbx polymorphism

The table argument is always 128-bit (and interpreted as <16 x i8>) so the
extra specifier for it is just clutter.

No user-visible behaviour change, so no tests.
AArch64: use less cluttered intrinsic for vtbl/vtbx

The table is always 128-bit so there's no reason to specify it every time we
want the intrinsic.