Add doxygen comments to emmintrin.h's intrinsics.

Add doxygen comments to emmintrin.h's intrinsics.

Only around 50% of the intrinsics in this file are 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 docu
ment.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276499 91177308-0d34-0410-b5e6-96231b3b80d8