Mutlblock pass that eliminates flag stores that are never read.
This is a sibling to RCLSE and will likely be absorbed by it when we add MB support there.
The Passmanager passes need very little or zero x86 knowledge to do
their work. They don't need the x86 OpDispatcher handling code at all.
Convert this entirely over to using the IREmitter class so it can
actually optimize IR code from the IRLoader frontend
Stefanos Kornilios Mitsis Poiitidis