patch 8.1.2120: some MB_ macros are more complicated than necessary

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.
diff --git a/src/popupmenu.c b/src/popupmenu.c
index 846dbc2..b88c653 100644
--- a/src/popupmenu.c
+++ b/src/popupmenu.c
@@ -1151,7 +1151,7 @@
 		}
 	    }
 	    item->cells += ptr2cells(p);
-	    p += MB_PTR2LEN(p);
+	    p += mb_ptr2len(p);
 	}
 	item->bytelen = p - item->start;
 	if (item->cells > max_cells)
@@ -1195,7 +1195,7 @@
 	    {
 		cells = item->indent * 2;
 		for (p = item->start + skip; p < item->start + item->bytelen;
-							    p += MB_PTR2LEN(p))
+							    p += mb_ptr2len(p))
 		    if ((cells += ptr2cells(p)) > BALLOON_MIN_WIDTH)
 			break;
 		thislen = p - (item->start + skip);