That's what a lot of JIT compilers do. SpiderMonkey for example does type inference in the interpreter (or baseline JIT? what do they use now) and uses that to

That's what a lot of JIT compilers do. SpiderMonkey for example does type inference in the interpreter (or baseline JIT? what do they use now) and uses that to generate almost exactly what you're describing. If the type inference holds (it usually does, even in dynamic languages) then you get almost native performance.