Short circuit rate_block in block_rd_txfm.

Don't run rate_block (cost_coeffs) if distortion alone is enough to
surpass best_rd.

This decreases 2nd pass runtime on HD at speed 2 by about 2%. There is
zero effect on output if tx_cache is removed.

Change-Id: Ia3b1cc77bfbe6ee988c395fde06c0eb92940b784
1 file changed