Optimizer code cleanups, no logic changes

- Updated comments
- Added some extra DEBUG
- Indentation changes and break long lines
- Trivial code changes like:
  - Combining 2 statements in one
  - Reorder DBUG lines
  - Use a variable to store a pointer that is used multiple times
- Moved declaration of variables to start of loop/function
- Removed dead or commented code
- Removed wrong DBUG_EXECUTE code in best_extension_by_limited_search()

sql/sql_select.h

@@ -1343,7 +1343,9 @@ public:
   int dbug_join_tab_array_size;
 #endif
 
-  /* We also maintain a stack of join optimization states in * join->positions[] */
+  /*
+    We also maintain a stack of join optimization states in join->positions[]
+  */
 /******* Join optimization state members end *******/
 
   /*