[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.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298371 91177308-0d34-0410-b5e6-96231b3b80d8