fix spurious 'Use of reference' warning

My recent OP_MULTIDEREF addition introduced a bug where, when
converting a constant array index into a UV, it checked for a ref
and issued a "Use of reference "HASH(0x7fd190915ba8)" as array index"
warning *before* it had confirmed that the OP_CONST was the only op in
the index expression. So things like

    use constant HASHREF => { a => 1 };
    () = $_[HASHREF->{a} ];

would generate two spurious warnings.

The fix is easy. Only test for the warning on the second pass;
we'll already have abandoned the optimisation attempt on the first pass
if the index expression isn't a simple constant.