Fix a bug in xmmintrin.h.
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@209489 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang