Move bit_reader after cb_buffer_base in ThreadData

The xd and cb_buffer_base fields of struct ThreadData have 32 or
16-byte alignment requirements. Move the declaration of bit_reader
after those two fields, which may reduce the padding for alignment.

Change-Id: I2e3e86bb66a29de03fb9729212e56d80b097b2b4
diff --git a/av1/decoder/decoder.h b/av1/decoder/decoder.h
index 467c8a2..685c931 100644
--- a/av1/decoder/decoder.h
+++ b/av1/decoder/decoder.h
@@ -48,9 +48,9 @@
                                                    MACROBLOCKD *const xd);
 
 typedef struct ThreadData {
-  aom_reader *bit_reader;
   DECLARE_ALIGNED(32, MACROBLOCKD, xd);
   CB_BUFFER cb_buffer_base;
+  aom_reader *bit_reader;
   uint8_t *mc_buf[2];
   int32_t mc_buf_size;
   int mc_buf_use_highbd;  // Boolean: whether the byte pointers stored in