Skip unnecessary computation in search_txk_type()

No need to reconstruct a intra tx block if it's the last one in the
macroblock.

On speed 1, improve keyframe encoding speed by 15%; overall encoding
by 2.5%.

Change-Id: Ie109c2e0be59b918c226b24c7ded6cf8e1ee06e0
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 0709a44..9ebf0e2 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -2044,7 +2044,9 @@
   }
 
 RECON_INTRA:
-  if (!is_inter && best_eob) {
+  if (!is_inter && best_eob &&
+      (blk_row + tx_size_high_unit[tx_size] < mi_size_high[plane_bsize] ||
+       blk_col + tx_size_wide_unit[tx_size] < mi_size_wide[plane_bsize])) {
     // intra mode needs decoded result such that the next transform block
     // can use it for prediction.
     if (cpi->sf.optimize_coefficients != FULL_TRELLIS_OPT) {