math/big: add more complete tests and benchmarks of assembly

Also fix a few real but currently harmless bugs from CL 664895.
There were a few places that were still wrong if z != x or if a != 0.

Change-Id: Id8971e2505523bc4708780c82bf998a546f4f081
Reviewed-on: https://go-review.googlesource.com/c/go/+/664897


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>