Don't use an out register to hold the vector number across the call of the interrupt handler in intr_fast() as the handler might clobber it (no in-tree handler currently does but an upcoming one will). While at it, tidy the register usage in the interrupt counting code.
Don't use an out register to hold the vector number across the call
of the interrupt handler in intr_fast() as the handler might clobber
it (no in-tree handler currently does but an upcoming one will).
While at it, tidy the register usage in the interrupt counting code.

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