Fix BTB not being refilled on some indirect jumps

We are overloading the BTB-hit signal to mean that any part of the frontend
changed the control-flow, not just the BTB.  That's the right thing to do for
most of the control logic, but it means the BTB sometimes won't get refilled
when we'd like it to.  This commit makes the frontend use an invalid BTB entry
number when it, rather than the BTB, changes the control flow.  Since the
entry number is invalid, the BTB will treat it as a miss and refill itself.

This is kind of a hack, but a more palatable fix requires reworking the RVC
IBuf, which I don't have time for right now.