DDG: add some handy methods.
DDG: add some handy methods.

f-prettyland/angr,chubbymaggie/angr,axt/angr,chubbymaggie/angr,angr/angr,iamahuman/angr,axt/angr,tyb0807/angr,tyb0807/angr,chubbymaggie/angr,angr/angr,f-prettyland/angr,f-prettyland/angr,iamahuman/angr,iamahuman/angr,schieb/angr,schieb/angr,tyb0807/angr,schieb/angr,axt/angr,angr/angr