C++: iterated dominance frontier algorithm for IR

Use the iterated dominance frontier algorithm to speed up dominance
frontier calculations. The implementation is copied from d310338.

Before this change, the SSA calculations for unaliased and aliased SSA
used 169.9 seconds in total on these predicates:

    7:Dominance::getDominanceFrontier#2#ff .. 49s
    7:Dominance::blockDominates#2#ff ........ 47.5s
    8:Dominance::getDominanceFrontier#ff .... 44.4s
    8:Dominance::blockDominates#ff .......... 29s

After this change, the above predicates are replaced by two copies of
`getDominanceFrontier`, each of which takes less than a second.

Author: jbj

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/reachability/Dominance.qll

@@ -11,11 +11,12 @@ predicate blockDominates(Graph::Block dominator, Graph::Block block) {
   blockStrictlyDominates(dominator, block) or dominator = block
 }
 
-pragma[noinline]
 Graph::Block getDominanceFrontier(Graph::Block dominator) {
-  exists(Graph::Block pred |
-    Graph::blockSuccessor(pred, result) and
-    blockDominates(dominator, pred) and
-    not blockStrictlyDominates(dominator, result)
+  Graph::blockSuccessor(dominator, result) and
+  not blockImmediatelyDominates(dominator, result)
+  or
+  exists(Graph::Block prev | result = getDominanceFrontier(prev) |
+    blockImmediatelyDominates(dominator, prev) and
+    not blockImmediatelyDominates(dominator, result)
   )
 }

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/internal/reachability/Dominance.qll

@@ -11,11 +11,12 @@ predicate blockDominates(Graph::Block dominator, Graph::Block block) {
   blockStrictlyDominates(dominator, block) or dominator = block
 }
 
-pragma[noinline]
 Graph::Block getDominanceFrontier(Graph::Block dominator) {
-  exists(Graph::Block pred |
-    Graph::blockSuccessor(pred, result) and
-    blockDominates(dominator, pred) and
-    not blockStrictlyDominates(dominator, result)
+  Graph::blockSuccessor(dominator, result) and
+  not blockImmediatelyDominates(dominator, result)
+  or
+  exists(Graph::Block prev | result = getDominanceFrontier(prev) |
+    blockImmediatelyDominates(dominator, prev) and
+    not blockImmediatelyDominates(dominator, result)
   )
 }