PPoPP2016-mutable-fields-fine: Fix GC for TRec mutable array entries.

We use the offset to determine if an entry is a pointer.  With mutable arrays
we have pointer entires beyond the index of the last non-pointer.

PPoPP2016-mutable-fields-hybrid: Fix GC for TRec mutable array entries.

We use the offset to determine if an entry is a pointer.  With mutable arrays
we have pointer entires beyond the index of the last non-pointer.