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/search.c b/src/search.c
index 0aebea4..42351d5 100644
--- a/src/search.c
+++ b/src/search.c
@@ -421,7 +421,7 @@
 
     if (ic && !no_smartcase && scs
 #ifdef FEAT_INS_EXPAND
-				&& !(ctrl_x_mode && curbuf->b_p_inf)
+			     && !(ctrl_x_mode_not_default() && curbuf->b_p_inf)
 #endif
 								    )
 	ic = !pat_has_uppercase(pat);