[CIR] Fix atomic-fetch conflict: (#199066)
We had #195537 and #198871 both attempt to fix the same problem, in
mildly different ways. My patch (the latter) did so in a way that was
rendered 'moot' by the former patch.
After looking into it further, I believe that the former is the
'correct' patch.
THIS patch is a partial revert of my patch, leaving the lambda (as I
think it is a good 'readability' improvement), plus leaving/updating
the test.
