nextPc only reflected things known in the Decode stage, but not before
(i.e. BTB predictions).
This meant that we were pretty much always redirecting JRs.
I needed to pipe through whether a jump was predicted by the BTB or not,
and then only stop predicting if there is no nextPc prediction and also
no BTB prediction. This should actually reasonably approximate SBC.
Also, this one is not "redirecting", it's just flushing (flipping
decode_epoch_local), trusting that we'll get a later redirection.
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