Revert "use rb_iseq_line_no()."

This reverts commit f4b590a.

ext/ruby_debug/ruby_debug.c

@@ -751,8 +751,28 @@ set_thread_event_flag_i(st_data_t key, st_data_t val, st_data_t flag)
 static int
 find_prev_line_start(rb_control_frame_t *cfp)
 {
-    const rb_iseq_t *iseq = cfp->iseq;
-    return rb_iseq_line_no(iseq, cfp->pc - iseq->iseq_encoded - 1);
+    int i, pos, line_no;
+    pos = cfp->pc - cfp->iseq->iseq_encoded;
+    for (i = 0; i < cfp->iseq->insn_info_size; i++)
+    {
+        if (cfp->iseq->insn_info_table[i].position != pos)
+            continue;
+
+        if (i == 0) return(0); // TODO
+
+        pos = -1;
+        line_no = cfp->iseq->insn_info_table[--i].line_no;
+        do {
+            if (cfp->iseq->insn_info_table[i].line_no == line_no)
+                pos = cfp->iseq->insn_info_table[i].position;
+            else
+                break;
+            i--;
+        } while (i >= 0);
+
+        return(pos);
+    }
+    return(-1);
 }
 
 static rb_control_frame_t *