Fix bug #78271

Fix bug #78271

When cleaning nops in the dfa pass, we were always keeping the
smart branch inhibiting nop that occurs directly before the jump
instruction. However, as we skip unreachable blocks entirely, it
may happen that we need to keep a nop that occurs further back,
prior to the unreachable blocks. Account for that case now.

We should really do something about the smart branch situation,
this is very fragile...