Fix unary/binary ops with OPpTARGET_MY flag.

For expressions like:

  $x = $a + $b;
  $x = abs($y);

where the result is assigned to a lexical variable, the sassign/padsv
is optimized away and the target for the unary/binary op contains the
pad index of the lexical; at the AST level it's simpler to synthesize
an assignment node.