Tweak *mmintrin.h so that they don't make any bad assumptions about alignment...

Tweak *mmintrin.h so that they don't make any bad assumptions about alignment (which probably has little effect in practice, but better to get it right).  Make the load in _mm_loadh_pi and _mm_loadl_pi a single LLVM IR instruction to make optimizing easier for CodeGen.

rdar://10054986

llvm-svn: 139874