Add doxygen comments to xmmintrin.h's intrinsics.

Only half of the intrinsics in this file is documented here. The patch for the o
ther half will be sent out later.

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.

llvm-svn: 272121