cmd/compile: replace magic numbers "2" and "1" with named constant
This was originally done for a #next-encoding-based check for
misbehaving loops, but it's a good idea anyhow because it makes
the code slightly easier to follow or change (we may decide to
check for errors the "other way" anyhow, later).
Change-Id: I2ba8f6e0f9146f0ff148a900eabdefd0fffebf8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/540261
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>