Refactor setup_segmentation_dequant.
Refactor setup_segmentation_dequant.

* Deduplicate segmentation on and off paths
* Declare variables late

Change-Id: If0f7fcce2b10d9352c056456596f77bb14ba16ec

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