list assign in list context was over-optimising

list assign in list context was over-optimising

GH #17816

This code:

    my $x = 1;
    print (($x, undef) = (2 => $x));

was printing "22" when it should have been printing "21".
An optimisation skips the 'common values on both sides' test
when the LHS of an assign only contains a single var; as the example
above shows, this is not sufficient.

This was broken by v5.23.1-202-g808ce55782

This commit fixes it by counting undef's on the LHS towards the var
count if they don't appear first.