Simplify and fix runahead computation. #6712

Job	Run time
1/4	13m 5s
2/4	14m 8s
3/4	14m 23s
4/4	14m 45s
py-3.11	13m 55s
flaky	8m 26s
_remote_background_indep_poll	5m 25s
_remote_background_indep_tcp	7m 0s
macos 1/5	21m 55s
macos 2/5	21m 1s
codecov	6s
	2h 14m 9s

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