cmd/compile/internal/typecheck: add NewMethodExpr helper

cmd/compile/internal/typecheck: add NewMethodExpr helper

This CL refactors the common pattern for constructing OMETHEXPR nodes,
which is the most common use of ir.TypeNode currently.

Change-Id: I446a21af97ab5a4bc2f04bbd581c1ede8a5ede60
Reviewed-on: https://go-review.googlesource.com/c/go/+/520978
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>