6) References can only go to lower memory addresses. That is, for each stack slot sp:
(sp->prev == NULL) or (sp->prev < sp)
+
+ 7) If an instruction produces any stackslots, iptr->dst points to the highest-address
+ stackslot produced
+ Corollary: if iptr->dst points to curstack or below, the instruction does
+ not produce any stackslots
+
ATTENTION: The instack of a block and the slots produced within the block
are *not* guaranteed to be adjacent (see figure below)!