Separate the last coeff decoding from the rest
Separate the last coeff decoding from the rest

This speed up av1_read_coeffs_txb by 10%
The over decoding time is reduced by 1.3%

BUG=aomedia:1369

Change-Id: Ic6e2f8dc4e04389678c7af9e742c31917c1c8df5

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