patch 8.0.1479: insert mode completion state is confusing

Problem:    Insert mode completion state is confusing.
Solution:   Move ctrl_x_mode into edit.c.  Add CTRL_X_NORMAL for zero.
diff --git a/src/getchar.c b/src/getchar.c
index 70cf4f1..39ccebb 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2115,7 +2115,8 @@
 			    && State != ASKMORE
 			    && State != CONFIRM
 #ifdef FEAT_INS_EXPAND
-			    && !((ctrl_x_mode != 0 && vim_is_ctrl_x_key(c1))
+			    && !((ctrl_x_mode_not_default()
+						      && vim_is_ctrl_x_key(c1))
 				    || ((compl_cont_status & CONT_LOCAL)
 					&& (c1 == Ctrl_N || c1 == Ctrl_P)))
 #endif