MFC r266674:

Pull in r209489 from upstream clang trunk (by Akira Hatanaka):

  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>