MDEV-36125 Cleanup ahead of [NO_]INDEX_MERGE

Introduces code cleanup and small refactorings, such as:

  - Flip keys_to_use.is_clear_all() and de-dent
  - Rename best_trp to best_read_plan
  - Fix key_parts assignment spacing
  - find_shortest_key now keeps happy path to the left
  - Cleanup in the get_best_ror_intersect, remove_nonrange_trees,
    and restore_nonrange_trees functions
  - Reorganization of locals within test_quick_select
  - De-dent some large code blocks by changing
      if (key)
      {
        LOTS-OF-CODE;
      }
    into
      if (!key)
        continue;
      LOTS-OF-CODE;

Author: DaveGosselin-MariaDB

sql/opt_hints.cc

@@ -934,8 +934,9 @@ hint_state hint_table_state(const THD *thd,
 }
 
 
-bool hint_key_state(const THD *thd, const TABLE *table, uint keyno,
-                    opt_hints_enum type_arg, bool fallback_value)
+bool hint_key_state(const THD *thd, const TABLE *table,
+                    uint keyno, opt_hints_enum type_arg,
+                    bool fallback_value)
 {
   Opt_hints_table *table_hints= table->pos_in_table_list->opt_hints_table;