C++: Fix primitive_basic_block_member join order

This predicate looked like a join of two already-computed predicates,
but it was a bit more complicated because the `*` operator expands into
two cases: the reflexive case and the transitive case. The join order
for the transitive case placed the `PrimitiveBasicBlock` charpred call
_after_ the `member_step+` call, which means that all the tuples of
`member_step+` passed through the pipeline.

This commit changes the implementation by fully writing out the
expansion of `*` into two cases, where the base case is manually
specialised to make sure the join orderer doesn't get tempted into
reusing the same strategy for both cases. This speeds up the predicate
from 2m38s to 1s on a snapshot of our own C/C++ code.

Author: jbj

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

@@ -66,8 +66,14 @@ 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) {
-    pos = getMemberIndex(node) and
-    member_step*(bb, node)
+    primitive_basic_block_entry_node(bb) and
+    (
+      pos = 0 and
+      node = bb
+      or
+      pos = getMemberIndex(node) and
+      member_step+(bb, node)
+    )
   }
 
   /** Gets the number of control-flow nodes in the primitive basic block `bb`. */