simplify/rollback VLA pointer subtraction

simplify/rollback VLA pointer subtraction

I don't know if it makes a difference to gen_op(TOK_PDIV) or not,
but logically the ptr1_is_vla test in TP's VLA patch seems out of
order, where the patch to fix it would be:
------------------------------------------------------------------
@@ -1581,15 +1581,15 @@ ST_FUNC void gen_op(int op)
                 u = pointed_size(&vtop[-1].type);
             }
             gen_opic(op);
+            if (ptr1_is_vla)
+                vswap();
             /* set to integer type */
 #ifdef TCC_TARGET_X86_64
             vtop->type.t = VT_LLONG;
 #else
             vtop->type.t = VT_INT;
 #endif
-            if (ptr1_is_vla)
-                vswap();
-            else
+            if (!ptr1_is_vla)
                 vpushi(u);
             gen_op(TOK_PDIV);
         } else {
------------------------------------------------------------------

Instead of that patch, which increases the complexity of the code,
this one fixes the problem by just rolling back and retrying with
a simpler approach.