[dev.regabi] cmd/compile: use Ntype where possible

For nodes that are always a type expression, we can use Ntype instead
of Node.

Passes toolstash -cmp.

Change-Id: I28f9fa235015ab48d0da06b78b30c49d74c64e3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/280642


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