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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202258 91177308-0d34-0410-b5e6-96231b3b80d8