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Allow OPpTARGET_MY optimisation for x
Many operators have a special SV allocated in the pad which is used
for return values (the target). If we make that pad offset point to
a lexical variable, then we can optimise, say, $lexical = "$foo" into
just "$foo", where $lexical is stringify’s ‘target’. And pp_stringify
doesn’t need to know any better. We already do that for many ops.
This can be extended to x. Despite what the comment in op_private
says, list return values do not matter here, because the OPpTARGET_MY
optimisation only happens when the operator is in scalar context. And
the scalar code paths use TARG and push TARG on to the stack.