Don't iterate through the bits of the pending interrupt register if the
whole register is zero.  Most of the registers will be zero most of the time.
Do not EOI an interrupt until the point after the filter handlers / before
threaded handlers.

It's not easy to see from the diffs of this change exactly how it
accomplishes the above.  The arm_mask_irq() and arm_unmask_irq() functions
are, respectively, the pre_thread and post_thread hooks.  Not seen in
these diffs, the arm_post_filter() routine also EOIs.  The post_filter
routine runs after filter handlers if there will be no threaded handlers,
so it just EOIs.  The pre_thread routine masks the interrupt (at the
controller, not the source) and EOIs.  So one way or another, the EOI
happens at the point where filter handlers are done.