Refactor trellis optimization process

This commit refactors the trellis coefficient optimization process.
It saves multiplications used to generate the final dequantized
coefficients. It also removes two memset operations on quantized
and dequantized coefficient sets.

The trellis coefficient optimization is on average running over
10% faster.

Change-Id: If3aa26d2a706c3012bf2b7ac059bf1825250e81f