* SimpleBusUL is used for MMIO and SimpleBusUH is used for memory
* should refactor SimpleBus2AXI4Converter to support SimpleBusUL and
  SimpleBusUH

This reverts commit 89b7c53e.

* Some small functions will return quickly. `ret` is executed without
  the correct return address pushed into RAS.

* ultraZ gives better performance than zedboard

* now wbData can change anytime

* no need to mask src1ForwardNextCycle with exu.io.out.fire(), since isu
  will be blocked until exu.io.out.fire()

* no forwarding, no branch prediction

* division should still trap