Common vars check should account for OPpTARGET_MY

When checking for common vars on either side of a list assignment (to
see whether we need to make temporary copies), we need to account for
the OPpTARGET_MY optimisation which transforms ‘$lex = $foo . $bar’
into a simple ‘$foo . $bar’ that writes directly to $lex instead of
writing to a scratch variable and returning that.

This was printing foofoo instead of foobar:

$ ./perl -Ilib -le 'my ($a,$b); $foo="foo"; $bar="bar"; ($a,$b) = ($b=$foo."", $a=$bar.""); print $a,$b'