Fix a bug in xmmintrin.h.

The last step of _mm_cvtps_pi16 should use _mm_packs_pi32, which is a function
that reads two __m64 values and packs four 32-bit values into four 16-bit
values.  

<rdar://problem/16873717>


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209489 91177308-0d34-0410-b5e6-96231b3b80d8