C++: Avoid outputting a cartesian product in AliasedSSA's getOverlap.

Author: geoffw0

cpp/ql/lib/semmle/code/cpp/ir/implementation/aliased_ssa/internal/AliasedSSA.qll

@@ -688,6 +688,11 @@ private Overlap getExtentOverlap(MemoryLocation0 def, MemoryLocation0 use) {
     (
       // EntireAllocationMemoryLocation exactly overlaps itself.
       use instanceof EntireAllocationMemoryLocation and
+      (
+        def.getAnAllocation() = use.getAnAllocation() or
+        not exists(def.getAnAllocation()) or
+        not exists(def.getAnAllocation())
+      ) and
       result instanceof MustExactlyOverlap
       or
       not use instanceof EntireAllocationMemoryLocation and