supertx: code refactoring + resolve conflicts with baseline

Refactoring: split prediction+extension for each plane, so we can
handle luma/chroma supertx pred in different ways.
Compatibility fix: fix conflicts with cb4x4 and chroma_sub8x8, now
for chroma sub8x8 supertx, only the top-left(basic cb4x4) or the
the bottom-right(cb4x4 + chroma_sub8x8) predictor will be used
without any blending within a 8x8 unit.

Change-Id: I6cf7b12768a82d3c7e01811ada02de84af9bd8ac
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 32ed8df..0c21cac 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -1455,7 +1455,9 @@
   int64_t rd1, rd2, rd;
   RD_STATS this_rd_stats;
 
+#if !CONFIG_SUPERTX
   assert(tx_size == get_tx_size(plane, xd));
+#endif  // !CONFIG_SUPERTX
 
   av1_init_rd_stats(&this_rd_stats);