Clean up this interface:

Clean up this interface:
1. Group similar methods into doxygen groups
2. Reorganize the groups into a consist flow.
3. Significantly improve the quality of the documentation on several methods
4. Rewrite srem and sdiv to eliminate a copy and improve readability.
5. Eliminate unneeded forward references.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35300 91177308-0d34-0410-b5e6-96231b3b80d8