Previously we were relying on the beat count registers being exactly the
right number of bits such that we'd overflow from 7 back to 0 after the
final flit. This change aligns the LLC adapter with the MMIO adapter,
which already does things in a safer way. We can also just look at rlast
for read respones rather than a full 3-bit comparison (the MMIO adapter
also makes this micro-optimisation).
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