Make metaops with assigning ops between 1.3x and 1.7x as fast
Make metaops with assigning ops between 1.3x and 1.7x as fast

If the left hand side of a meta op is a mutable structure, and we know
that the op does an assignment, we don't have to build any result structure
as the result is the same as the left hand side then.  This allows for quite
some savings.

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