[dist-8x8] Move asserts behind DEBUG_DIST_8X8 flag
[dist-8x8] Move asserts behind DEBUG_DIST_8X8 flag

Change-Id: I0cd3de4a5673003f9fbe9c615e139e8133a99dcc

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