cmd/compile: make check2 gracefully exit if it reported errors

Otherwise, if -d=panic was set, check2 will treat already reported
error as internal compiler error.

For #43311
Fixes #44445

Change-Id: I5dbe06334666df21d9107396b9dcfdd905aa1e44
Reviewed-on: https://go-review.googlesource.com/c/go/+/294850


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