MDEV-18478 ANALYZE for statement should show selectivity of ICP, part#2

Part#2, variant 2: Make the printed r_ values in JSON output consistent.
After this patch, ANALYZE output has:

- r_index_rows (NEW) - Observed number of rows before ICP or Rowid Filtering
  checks. This is a per-scan average. like r_rows and "rows" are.

- r_rows (AS BEFORE) - Observed number of rows after ICP and Rowid Filtering.

- r_icp_filtered (NEW) - Observed selectivity of ICP condition.

- (AS BEFORE) observed selectivity of Rowid Filter is in
  $.rowid_filter.r_selectivity_pct

- r_total_filtered - Observed combined selectivity: fraction of rows left
  after applying ICP condition, Rowid Filter, and attached_condition.
  This is now comparable with "filtered" and is printed right after it.

- r_filtered (AS BEFORE) - Observed selectivity of "attached_condition".

Tabular ANALYZE output is not changed. Note that JSON's r_filtered and
r_rows have the same meanings as before and have the same meaning as in
tabular output.

sql/sql_explain.h

@@ -604,7 +604,7 @@ enum explain_extra_tag
 class EXPLAIN_BKA_TYPE
 {
 public:
-  EXPLAIN_BKA_TYPE() : join_alg(NULL) {}
+  EXPLAIN_BKA_TYPE() : join_alg(NULL), is_bka(false) {}
 
   size_t join_buffer_size;
 
@@ -616,6 +616,9 @@ public:
   */
   const char *join_alg;
 
+  /* true <=> BKA is used */
+  bool is_bka;
+
   /* Information about MRR usage.  */
   StringBuffer<64> mrr_type;
   
@@ -676,14 +679,6 @@ public:
 
   void print_explain_json(Explain_query *query, Json_writer *writer,
                           bool is_analyze);
-
-  /*
-    TODO:
-      Here should be ANALYZE members:
-      - r_rows for the quick select
-      - An object that tracked the table access time
-      - real selectivity of the filter.
-  */
 };
 
 
@@ -870,7 +865,7 @@ public:
   /*
     This is either pushed index condition, or BKA's index condition. 
     (the latter refers to columns of other tables and so can only be checked by
-     BKA code). Examine extra_tags to tell which one it is.
+     BKA code). Examine extra_tags (or bka_type.is_bka) to tell which one it is.
   */
   Item *pushed_index_cond;