math/big: arm64 assembly code for shlVU is incorrect [1.12 backport]

[gopherbot]

@erifan requested issue #31084 to be considered for backport to the next 1.13 minor release.

@gopherbot Please open backport issue to 1.13

[dmitshur]

No need for a cherry-pick for 1.13, the fix is already on master branch and will be a part of Go 1.13 when that's released.

See #31084 (comment) for context.

[dmitshur]

Targeting 1.12 and reopening for consideration per #31084 (comment).

[ianlancetaylor]

I think it's fine to backport https://golang.org/cl/169780 to the 1.12 branch.

I don't think it's a release blocker, as the problem is not new.

[toothrot]

Moving to Go1.12.8 because there is no backport CL ready in time for this release.

[gopherbot]

Change https://golang.org/cl/185041 mentions this issue: [release-branch.go1.12] math/big: fix the bug in assembly implementation of shlVU on arm64