Don't vectorize target-specific types (ppc_fp128, x86_fp80, etc.).

Target specific types should not be vectorized. As a practical matter,
these types are already register matched (at least in the x86 case),
and codegen does not always work correctly (at least in the ppc case,
and this is not worth fixing because ppc_fp128 is currently broken and
will probably go away soon).

llvm-svn: 155729