Cranelift: Consider emitting jump tables at the end of functions

Right now they are emitted inline right after the code that evaluates the table index and loads the target out of the table. This seems like it could waste instruction pipeline bandwidth and icache space. Might be better to emit them at the end of the function, similar to @alexcrichton's recent changes for emitting `MInst::TrapIf`'s trap instructions at the end of the function. Would need to integrate with the whole islands business, of course.