[DOXYGEN] Minor improvements in doxygen comments.

- I removed doxygen comments for the intrinsics that "alias" the other existing documented intrinsics and that only sligtly differ in spelling (single underscores vs. double underscores). 
 #define _tzcnt_u16(a)     (__tzcnt_u16((a)))

It will be very hard to keep the documentation for these "aliases" in sync with the documentation for the intrinsics they alias to. Out of sync documentation will be more confusing than no documentation.

I got an OK from Eric Christopher to commit doxygen comments without prior code
review upstream.

llvm-svn: 301652