Add doxygen comments to mmintrin.h's intrinsics.

The doxygen comments are automatically generated based on Sony's intrinsics docu
ment.

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@272350 91177308-0d34-0410-b5e6-96231b3b80d8