[DOXYGEN] Improvements to smmintrin.h and emmintrin.h intrinsics.

I made some small changes in smmintrin.h and emmintrin.h intrinsics.
 - changed some regular comments '//' into doxygen-style comments '///' where necessary
 - removed some trailing spaces in doxygen comments.

I got an OK from Eric Christopher to commit doxygen comments without prior code
review upstream.

llvm-svn: 298371