op.c:op_lvalue: Remove redundent case OP_RETURN

The default case does nothing if the type is OP_LEAVESUBLV.  This has
been so since 145b2bbb3.
Simplify OP_LEAVESUBLV logic in op.c:op_lvalue

If the lvalue type was OP_LEAVESUBLV, then the code for handling
entersub ops would go through extra checking to see whether we
have an lvalue sub available at compile time, before falling
through to the croaking code.  But the croaking code does not croak
if the type is OP_LEAVESUBLV, so the extra checks were for nothing.
We can skip those checks here just as we do for OP_GREPSTART,
OP_ENTERSUB and OP_REFGEN lvalue types.
Remove __SUB__ from Deparse-core.t exception list

This list is for keywords exempt from the checks that make sure all
keywords have been tested.  __SUB__ *is* tested, so it does not
belong here.
[perl #122771] Fix assertion failure with -d and sort

-d was conflicting with sort optimisations added by 932bca295d, which
had assertions that checked for nextstate, but not dbstate ops.
Make B::Deparse qualify sub calls named after keywords

While B::Deparse was correctly applying CORE:: as necessary to dis-
ambiguate between keywords and custom subroutines, it was not doing
likewise for subroutines whose names were keywords.  main::foo()
should be deparsed as main::foo() if ‘foo’ is a keyword.