Refactor quantization C code.
This commit de-duplicates C reference quantization code and unifies quantization matrix (QM) and non-QM code paths when there is no SIMD. The reorganisation also will facilitate re-using SIMD quant functions for QM when the matrix is flat, as is the default when AOM_QM is enabled. Change-Id: Idbfdac9eb9a31adcffe734aac1877d58b86fab77
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