Use memcpy for save/restore_predictor

The save_predictor and restore_predictor functions perform a 1D backup
of the 2D predictor block. Use memcpy to get a faster copy operation
than 4 individual load/stores.

Change-Id: Ia609ed71fbff1ade6fa677186efce9ee29167fd6

vp8/encoder/rdopt.c

@@ -589,12 +589,8 @@ static void save_predictor(unsigned char *predictor, unsigned char *dst)
     int r, c;
     for (r = 0; r < 4; r++)
     {
-        for (c = 0; c < 4; c++)
-        {
-            *dst = predictor[c];
-            dst++;
-        }
-
+        memcpy(dst, predictor, 4);
+        dst += 4;
         predictor += 16;
     }
 }
@@ -603,12 +599,8 @@ static void restore_predictor(unsigned char *predictor, unsigned char *dst)
     int r, c;
     for (r = 0; r < 4; r++)
     {
-        for (c = 0; c < 4; c++)
-        {
-            predictor[c] = *dst;
-            dst++;
-        }
-
+        memcpy(predictor, dst, 4);
+        dst += 4;
         predictor += 16;
     }
 }