Add more doxygen comments to emmintrin.h's intrinsics.

With this patch, 75% of the intrinsics in this file will be documented now. The patches for the rest of the intrisics in this file will be send out later.

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

I got an OK from Eric Christopher to commit doxygen comments without prior code review upstream. This patch was internally reviewed by Yunzhong Gao.

llvm-svn: 284754