[RegionInfo] Use RegionInfo* instead of RegionInfoPass* as graph type

This allows printing region graphs when only the RegionInfo (e.g. Region::getRegionInfo()), but no RegionInfoPass object is available.

Specifically, we will use this to print RegionInfo graphs in the debugger.

Differential version: http://reviews.llvm.org/D11874

Reviewed-by: grosser
llvm-svn: 244442