make sure MERGESORT-* don't leak low-level types
make sure MERGESORT-* don't leak low-level types

depending on how many items were in the list, you
could end up getting a BOOTIntArray instead of an
intarray, for example with 65 elements.

This was caused by `$A` and `$B` switching places after
every "round" of mergesort application, and if you
got the right number of rounds, you'd get `$B` instead,
which was just a nqp::list_i (or _n or _s).

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