Remove obsolete ObjCMethodDecl arg_type iterator functions

These were set into deprecation in r199773.
Improve descriptions of combiner-alias-analysis and combiner-global-alias-analysis
Add combiner-aa-only-func (debug only)

This option (which is !NDEBUG only) allows restricting the use of alias
analysis in DAGCombiner to a specific function. This has proved extremely
valuable to isolating bugs related to this feature, and mirrors the
misched-only-func option provided by the new instruction scheduler.