extend dropBogusSubstitutions to also prune junk RHS
extend dropBogusSubstitutions to also prune junk RHS

ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell,ucsd-progsys/liquidhaskell