Revert "internal/sync: optimize CompareAndSwap and Swap"
This reverts CL 606462.
Reason for revert: Breaks atomicity between operations. See #70970.
Change-Id: I1a899f2784da5a0f9da3193e3267275c23aea661
Reviewed-on: https://go-review.googlesource.com/c/go/+/638615
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com>