Per previous analysis, the most correct notion of SampleOverhead is that

Per previous analysis, the most correct notion of SampleOverhead is that
it is just the total time to do INSTR_TIME_SET_CURRENT(), and not any of
the other code involved in InstrStartNode/InstrStopNode.  Even though I
fear we may end up reverting this patch altogether, we may as well have
the most correct version in our CVS archive.