An invariant of AccessLists is that the defining access of a Use or Def is the nearest preceding Def node. For instance, within a BasicBlock:
0 = MemoryDef(liveOnEntry) 1 = MemoryDef(0) 2 = MemoryUse(n) 3 = MemoryDef(m)
1 is the nearest parent Def of 2 and 3, and m and n must be 1.
This patch simplifies the interfaces of createMemoryAccessBefore/After, and augments them to maintain this invariant.
Additionally, when inserting a new Def after an existing Def, there is currently no (clean) way to update the users of the old Def to use the new Def. So createDefiningAccess now permits the option of updating the users.