[MC] Fold A-B when A is a pending label or A/B are separated by a MCFillFragment

When the MCAssembler is non-null and the MCAsmLayout is null, we can fold A-B
in these additional cases:

* when A is a pending label (will be reassigned to a real fragment in flushPendingLabels())
* A and B are separated by a MCFillFragment with a constant size