Doxygen comments for avxintrin.h.

Added doxygen comments to avxintrin.h's intrinsics. As of now, around 75% of the
intrinsics in this file are documented here. The patches for the other 25% will be se
nt out later.

Removed extra spaces in emmitrin.h.

Note: 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.

llvm-svn: 286336