cmd/compile: fix Node.Etype overloading

Add helper methods that validate n.Op and convert to/from the
appropriate type.

Notably, there was a lot of code in walk.go that thought setting
Etype=1 on an OADDR node affected escape analysis.

Passes toolstash-check.

TBR=marvin

Change-Id: Ieae7c67225c1459c9719f9e6a748a25b975cf758
Reviewed-on: https://go-review.googlesource.com/99535


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>