Update PostponeDequantizeOpBelowUseChainPass cost model (#22813)

[DrJessop]

Summary:

If we move a dequant down and it happens to be right above a quant, there should be no additional cost incurred by the dequant. That is now reflected in the cost model.

Reviewed By: ethansfng

Differential Revision: D119727954

[pytorch-bot]

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