[dev.regabi] cmd/compile: use node walked flag to prevent double walk for walkSwitch

[dev.regabi] cmd/compile: use node walked flag to prevent double walk for walkSwitch

CL 283672 added a flag to prevent double walking, use that flag instead
of checking SwitchStmt.Compiled field.

Passes toolstash -cmp.

Change-Id: Idb8f9078412fb789f51ed4fc4206638011e38a93
Reviewed-on: https://go-review.googlesource.com/c/go/+/283733
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>