This commit cleans up exception vector usages in backend.

Previously the exception vector will go through the pipeline with the
uop. However, instructions with exceptions will enter ROB when they are
dispatched. Thus, actually we don't need the exception vector when an
instruction enters a function unit.

* exceptionVec, flushPipe, replayInst are reset when an instruction
enters function units.

* For execution units that don't have exceptions, we reset their output
exception vectors to avoid ROB to record them.

* Move replayInst to CtrlSignals.