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

With this patch, all intrinsics in this file (with an exception of a handful of a recently added ones) will be documented. I will send out a patch for 4 missining intrisics 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: 284934