BUG#920713: Wrong result (missing rows) with firstmatch+BNL, IN subquery, ...

- Disable use of join cache when we're using FirstMatch strategy, and the join
  order is such that subquery's inner tables are interleaved with outer.  Join 
  buffering code is incapable of handling such join orders.

- The testcase requires use of @@debug_optimizer_prefer_join_prefix to hit the bug, 
  but I'm pushing it anyway (including the mention of the variable in .test file), 
  so that it can be found and enabled when/if we get something comparable in the 
  main tree.

sql/sql_select.h

@@ -995,6 +995,13 @@ public:
   
   /* We also maintain a stack of join optimization states in * join->positions[] */
 /******* Join optimization state members end *******/
+
+  /*
+    Tables within complex firstmatch ranges (i.e. those where inner tables are
+    interleaved with outer tables). Join buffering cannot be used for these.
+  */
+  table_map complex_firstmatch_tables;
+
   /*
     The cost of best complete join plan found so far during optimization,
     after optimization phase - cost of picked join order (not taking into