Replace idct32x32_1024_add_ssse3 assembly with intrinsics

- Encoding/decoding test, BQTerrace_1920x1080_60.y4m, on
  i7-6700, no obvious user-level speed performance downgrade.
- Passed unit tests.

Change-Id: I20688e0dd3731021ec8fb4404734336f1a426bfc

test/partial_idct_test.cc

@@ -285,7 +285,7 @@ TEST_P(PartialIDctTest, SingleExtremeCoeff) {
   }
 }
 
-TEST_P(PartialIDctTest, DISABLED_Speed) {
+TEST_P(PartialIDctTest, Speed) {
   // Keep runtime stable with transform size.
   const int kCountSpeedTestBlock = 500000000 / input_block_size_;
   InitMem();