optimize postfix incr to prefix when unwanted
optimize postfix incr to prefix when unwanted

Somehow we never managed to do this obvious optimization even after we got
explicit knowledge of sink context from all the wanted/unwanted work.

labster/rakudo,softmoth/rakudo,salortiz/rakudo,MasterDuke17/rakudo,tony-o/rakudo,nbrown/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,tony-o/rakudo,Gnouc/rakudo,Gnouc/rakudo,tbrowder/rakudo,salortiz/rakudo,labster/rakudo,salortiz/rakudo,nbrown/rakudo,lucasbuchala/rakudo,Gnouc/rakudo,rakudo/rakudo,rakudo/rakudo,tbrowder/rakudo,lucasbuchala/rakudo,rakudo/rakudo,lucasbuchala/rakudo,tony-o/rakudo,labster/rakudo,lucasbuchala/rakudo,rakudo/rakudo,nbrown/rakudo,salortiz/rakudo,MasterDuke17/rakudo,labster/rakudo,MasterDuke17/rakudo,lucasbuchala/rakudo,tony-o/rakudo,MasterDuke17/rakudo,Gnouc/rakudo,nbrown/rakudo,MasterDuke17/rakudo,salortiz/rakudo,rakudo/rakudo,Gnouc/rakudo,tony-o/rakudo,softmoth/rakudo,softmoth/rakudo,labster/rakudo,rakudo/rakudo,salortiz/rakudo,tbrowder/rakudo,tbrowder/rakudo,nbrown/rakudo,softmoth/rakudo,softmoth/rakudo,Gnouc/rakudo,tbrowder/rakudo,labster/rakudo,tony-o/rakudo,nbrown/rakudo