[PowerPC] Enable vec_perm for long long and double vector types for VSX

VSX makes the "vector long long" and "vector double" types available.
This patch enables the vec_perm interface for these types.  The same
builtin is generated regardless of the specified type, so no
additional work or testing is needed in the back end.  Tests are added
to ensure this builtin is generated by the front end.

llvm-svn: 221988