math/big: apply simplification suggested in CL 73231 (or abandon)

[griesemer]

Reminder issue: Decide whether to apply https://go-review.googlesource.com/c/go/+/73231 for Go 1.15 (early) or abandon.

Specifically: The change must be applied to the new function lehmerSimulate. Also, need to verify that there's no performance regression.

[odeke-em]

Punting to backlog, please feel free to adjust it @griesemer.