C++: use plain recursion in PrimitiveBasicBlocks

It's sometimes faster but sometimes up to 2x slower to use plain
recursion here. On the other hand, plain recursion won't run out of Java
heap space, and it won't make unrelated computation slower by forcing
all RAM data out to disk.

Author: jbj

cpp/ql/src/semmle/code/cpp/controlflow/internal/PrimitiveBasicBlocks.qll

@@ -62,8 +62,14 @@ private cached module Cached {
 
   /** Holds if `node` is the `pos`th control-flow node in primitive basic block `bb`. */
   cached
-  predicate primitive_basic_block_member(Node node, PrimitiveBasicBlock bb, int pos) =
-    shortestDistances(primitive_basic_block_entry_node/1, member_step/2)(bb, node, pos)
+  predicate primitive_basic_block_member(Node node, PrimitiveBasicBlock bb, int pos) {
+    primitive_basic_block_entry_node(bb) and node = bb and pos = 0
+    or
+    exists(Node prev |
+      member_step(prev, node) and
+      primitive_basic_block_member(prev, bb, pos - 1)
+    )
+  }
 
   /** Gets the number of control-flow nodes in the primitive basic block `bb`. */
   cached