math/big: revisit alias checks in divLarge

[griesemer]

Reminder issue.

In the discussion of https://go-review.googlesource.com/c/go/+/78995, Brian Kessler suggested several cleanups, specifically the removal of the alias checks from divLarge (which is an internal function with all the callers being in control of the arguments).

[ianlancetaylor]

Ping @griesemer : roll forward to 1.12?

[gopherbot]

Change https://golang.org/cl/123196 mentions this issue: math/big: refine alias checks in div and divLarge

[gopherbot]

Change https://golang.org/cl/130775 mentions this issue: math/big: streamline divLarge initialization