cmd/compile/internal/typecheck: simplify NewMethodExpr

cmd/compile/internal/typecheck: simplify NewMethodExpr

This CL changes NewMethodExpr to directly construct the OMETHEXPR
node, instead of running through the generic OXDOT typechecking
machinery.

Change-Id: Ic2af0bab6ff1aef45e8463bccb1f69c50db68f65
Reviewed-on: https://go-review.googlesource.com/c/go/+/520919
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>