C++: Revert peer review suggestion.

The suggested change has a severe impact on row counts, as cpp does not cache
the results for `bbDominates`. Since the `getGuardedUpperBound` predicate the
cost of runtime complexity is considered higher than the benefit of this change.

Author: andersfugmann

cpp/ql/lib/semmle/code/cpp/rangeanalysis/SimpleRangeAnalysis.qll

@@ -1546,13 +1546,8 @@ private float getGuardedUpperBound(VariableAccess guardedAccess) {
     // node is attached to the block at the end of the edge and not on
     // the actual edge. It is therefore not possible to determine which
     // edge the guard phi node belongs to. The predicate below ensures
-    // that any predecessor other than the guard block is dominated by the
-    // guard phi node.
-    forall(BasicBlock pred |
-      pred = def.(BasicBlock).getAPredecessor() and pred != guard.getBasicBlock()
-    |
-      bbDominates(def, pred)
-    ) and
+    // that there is one predecessor, albeit somewhat conservative.
+    exists(unique(BasicBlock b | b = def.(BasicBlock).getAPredecessor())) and
     guardedAccess = def.getAUse(v) and
     result = max(float ub | upperBoundFromGuard(guard, guardVa, ub, branch))
   )

cpp/ql/test/library-tests/rangeanalysis/SimpleRangeAnalysis/upperBound.expected

@@ -598,7 +598,7 @@
 | test.c:671:9:671:9 | y | 2147483647 |
 | test.c:675:7:675:7 | y | 2147483647 |
 | test.c:684:7:684:7 | x | 2147483647 |
-| test.c:689:7:689:7 | x | 9 |
+| test.c:689:7:689:7 | x | 15 |
 | test.cpp:10:7:10:7 | b | 2147483647 |
 | test.cpp:11:5:11:5 | x | 2147483647 |
 | test.cpp:13:10:13:10 | x | 2147483647 |