🐛 Fixed ELW replay mechanism.

Created by: davideschiavone

To fix issue #160 (closed), the IRQ_FLUSH state when reached by the DECODE stage, was making the instruction valid for the next cycle. This state was reachable also by the ELW_EXE, that was using it to proceed with the next instruction once the event has completed (clock enabled). The DECODE state was considering the instruction valid due to #160 (closed), and so was reexecuting the ELW rather than the next instruction. Now a new state reachable only by the ELW_EXE state has been added to differentiate the meaning of the 2 situations.