Besides the improved coverage, this uncovered a few off-by-ones w.r.t.
IF writes, and IRQ ACK timing.
The IF write adjustment is a one-liner that has as consequence that mode
0 IRQs for lines with particular timing (e.g. due to scx offsets and/or
object positions) can be overwritten at up to one instruction earlier
than previously.
The ACK-related adjustments have consequence for cases when the same IRQ
is reasserted shortly after it has effected an interrupt in the sense
that, for the affected IRQs, the time window for when this will be
ignored is up to one instruction longer.
