Skip instructions on core flush in RSes

[piotro888]

Implement solution 3. from discussion on Speculation Backend: #557 (comment)

Would make solution progress in #498 - but works only for waiting operations in FUs that use RS.
Separate optimization would be needed (and complexity trade-off estimated) to cancel operations currently in flight/
Also similar mechanism needs to be implemented for LSU that defines its own internal RS. Implementing it in CSRUnit is probably not worth it, because it only waits for precommit -> only 1 additional cycle delay from backend.