[dev.regabi] cmd/compile: report unused variables during typecheck

Unused variables are a type-checking error, so they should be reported
during typecheck rather than walk.

One catch is that we only want to report unused-variable errors for
functions that type check successfully, but some errors are reported
during noding, so we don't have an easy way to detect that
currently. As an approximate solution, we simply check if we've
reported any errors yet.

Passes toolstash -cmp.

Change-Id: I9400bfc94312c71d0c908a491e85c16d62224c9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/280973


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>