[DOXYGEN] Improved doxygen comments for x86 intrinsics headers.

[DOXYGEN] Improved doxygen comments for x86 intrinsics headers.

Tagged instruction names with <c> INSTR_NAME </c> to display them in typewriter font.

In the past, \c command was used, unfortunately it applied to only one word.
<c> .. </c> has the same meaning, but applies to all words in between the tags.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289249 91177308-0d34-0410-b5e6-96231b3b80d8