cmd/compile/internal/ir: remove Ntype

cmd/compile/internal/ir: remove Ntype

This type used to provide extra type safety around which syntactic
nodes could also represent types, but now the only remaining use is
ir.TypeNode, and it always ends up as an ir.Node anyway. So we might
as well use Node instead.

Change-Id: Ia0842864794365b0e155dc5af154c673ffa2967b
Reviewed-on: https://go-review.googlesource.com/c/go/+/520609
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>