cmd/compile: use "pw" as receiver name consistently

cmd/compile: use "pw" as receiver name consistently

Change-Id: I912f1326fa442098065b62b7b0ef9048771e4701
Reviewed-on: https://go-review.googlesource.com/c/go/+/506476
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>