Let's go see some figures!
"Empirical Study of Power Consumption of x86-64 Instruction Decoder"
https://www.usenix.org/system/files/conference/cooldc16/cool...
From the conclusion:
"The result demonstrates that the decoders consume between 3% and 10% of the total processor package power in our benchmarks. The power consumed by the decoders is small compared with other components such as the L2 cache, which consumed 22% of package power in benchmark #1. We conclude that switching to a different instruction set would save only a small amount of power since the instruction decoder cannot be eliminated completely in modern processors."