Refactor tx_size step use cases in decoder
Refactor tx_size step use cases in decoder

Use lookup table to replace the arithmetic computation for transform
block step.

Change-Id: I1318d81bda9d7ffaf9d550acd19354b0615ede36

smarter/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,mbebenita/aom,smarter/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom