cmd/compile: fix importing of method expressions

For OMETHEXPR, the Name in the Selection needs to be properly
linked up to the method declaration. Use the same code we
already have for ODOTMETH and OCALLPART to do that.

Fixes #45503

Change-Id: I7d6f886d606bae6faad8c104f50c177f871d41c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/309831


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dan Scales <danscales@google.com>