[PPC] Add long long/double support for vec_cts, vec_ctu and vec_ctf

Add long long/double support for vec_cts, vec_ctu and vec_ctf.

Similar to this change in GCC:
https://gcc.gnu.org/ml/gcc-patches/2014-08/msg02653.html

Patch by Tim Shen.

llvm-svn: 257135