correct cost for implicit bit in mvs

Use 0xFFF0 vice 240 (0xF0) for determining whether the sometimes
implicit bit 3 will be transmitted. This is consistent with the decoder
and encode_mvcomponent().

Change-Id: Ic1304d0ab56844bed8236edd1c5243a6767fc6b1

vp8/encoder/encodemv.c

@@ -128,7 +128,7 @@ static unsigned int cost_mvcomponent(const int v, const struct mv_context *mvc)
 
         while (--i > 3);
 
-        if (x & 240)
+        if (x & 0xFFF0)
             cost += vp8_cost_bit(p [MVPbits + 3], (x >> 3) & 1);
     }