cmd/compile: fix case where init info of OAS node is dropped

When an OAS node is converted to an OSELRECV2 node in tcSelect(), the
possible DCL node in the Init field was being dropped, since a
completely new node was being created and the Init field was not set. I
don't expect n.Init() to be set for the ORECV case, but the code now
deals with that too.

Fixed bug in both tcSelect() and transformSelect().

Fixes #48289

Change-Id: I09918a70f7cbaa4aa9a17546169f908a8787df15
Reviewed-on: https://go-review.googlesource.com/c/go/+/348569


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