Cache METAOP_ASSIGN ops
Cache METAOP_ASSIGN ops

Not to specifically to make it faster, but to be able to assign a name
to the generated block without incurring a runtime penalty for that.
The name will allow metaoperators to introspect and optimize >>+=>> with
about a factor of 2.

Pushed as a separate commit for bisecting.

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