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_join_cache.cc

@@ -1080,7 +1080,7 @@ int JOIN_CACHE::init(bool for_explain)
 
 
 /* 
-  Check the possibility to read the access keys directly from the join buffer       
+  Check the possibility to read the access keys directly from the join buffer
   SYNOPSIS
     check_emb_key_usage()