optimize_b_greedy: Fix for TX64X64.
optimize_b_greedy: Fix for TX64X64.

Change-Id: Iff2780944e954973b9ee9ab3c04d1e6b44a7810d

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom