Add mderef uoop for loops also

[rurban]

So far we only optimize access to shaped arrays with const indices (#126), but we should extend that to loops with padsv and gvsv indices also, where the very first index can get a MDEREF_INDEX_uoob flag added, analog to aelem_u.

Such multideref ops appear pretty often even with single indices. E.g. in
my @b=(0..4); for (0..$#b) { $b[$_] = 0; } $b[$_] is converted
to a multideref, which previously was not optimized.

Mark that uoob index in dump (multideref_stringify) somehow. I used a " _u" suffix, i.e. [$i _u], to match the aelem_u, aelemfast_lex_u ops.