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/search.c b/src/search.c
index 63171a9..a0ca8df 100644
--- a/src/search.c
+++ b/src/search.c
@@ -2078,7 +2078,7 @@
 		    find_mps_values(&initc, &findc, &backwards, FALSE);
 		    if (findc)
 			break;
-		    pos.col += MB_PTR2LEN(linep + pos.col);
+		    pos.col += mb_ptr2len(linep + pos.col);
 		}
 		if (!findc)
 		{
@@ -2657,14 +2657,14 @@
 	if (PTR2CHAR(p) == c && (curwin->w_p_rl ^ p_ri))
 	    break;
 #endif
-	p += MB_PTR2LEN(p) + 1;
+	p += mb_ptr2len(p) + 1;
 	if (PTR2CHAR(p) == c
 #ifdef FEAT_RIGHTLEFT
 		&& !(curwin->w_p_rl ^ p_ri)
 #endif
 	   )
 	    break;
-	p += MB_PTR2LEN(p);
+	p += mb_ptr2len(p);
 	if (*p == NUL)
 	    return;
     }
@@ -5633,7 +5633,7 @@
 		    && action == ACTION_EXPAND
 		    && !(compl_cont_status & CONT_SOL)
 		    && *startp != NUL
-		    && *(p = startp + MB_PTR2LEN(startp)) != NUL)
+		    && *(p = startp + mb_ptr2len(startp)) != NUL)
 		goto search_line;
 	}
 	line_breakcheck();