Optimize backwardreferences

Optimize backwardreferences (about 0.1% byte savings) with almost same
compression speed (3% faster on defaut compression settings).
1.) Simplified iteration logic for HashChainFindCopy.
    - Remapped the iter_max constant.
2.) Simplified main for loop for BackwardReferencesHashChain
    - Removed 'if' conditions for corner cases in the main loop.
    - Refactored the method(AddSingleLiteral) for adding one pixel.

Change-Id: I1bc44832fd81f11e714868a13e606c8f83157e64