Allow OPpTARGET_MY optimisation for vec

Allow OPpTARGET_MY optimisation for vec

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 vec().  Despite what the comment in op_pri-
vate says, lvalue usage does not matter here, because the OPpTARGET_MY
optimisation only happens when the operator is in rvalue context.  And
the rvalue code paths use TARG and push TARG on to the stack.