Nvidia's Project Denver also attempted something similar. It shipped in the form of the HTC Nexus 9, but it wasn't any faster than the competition in benchmarks (where you'd expect a JIT to shine). In the real world it's even worse because JITs are not good for responsiveness, particularly JITs that are not scheduled by the kernel and are unable to be scheduled behind important work. The CPU just randomly stopped running code to run a JIT instead.

But I was more just talking about things like branch predictors. You can be informed by runtime data without being a full on JIT, which is the world very nearly every modern CPU lives in these days.

It was the idea behind HP Dynamo.

There's a good article here spoiled by the bitrotted formatting: http://archive.arstechnica.com/reviews/1q00/dynamo/dynamo-1.... It looks better if you use Firefox Reader view.