nilcheckelim2 cleans up by copying b.Values in a loop, omitting
OpUnknowns. However, the common case is that there are no OpUnknowns,
in which case we can skip a lot of work.

So we track the first nilcheck which was eliminated, if any, and only
start copying from there. If no nilcheck was eliminated we won't copy at all.

Fixes #20964

Change-Id: Icd44194cf8ac81ce6485ce257b4d33e093003a40
Reviewed-on: https://go-review.googlesource.com/65651
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>