MFC: r200938
MFC: r200938

- Don't check for a valid interrupt controller on every interrupt
  in intr_execute_handlers(). If we managed to get here without an
  associated interrupt controller we have way bigger problems.
  While at it predict stray vector interrupts as false as they are
  rather unlikely.
- Don't blindly call the clear function of an interrupt controller
  when adding a handler in inthand_add() as interrupt controllers
  like the one driven by upa(4) are auto-clearing and thus provide
  NULL instead.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase