add some logging for BackwardSlice class
add some logging for BackwardSlice class

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