[release-branch.go1.17] 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().

Cherry-picked from https://go-review.googlesource.com/c/go/+/348569

Fixes #49511

Change-Id: Id5b736daa8e90afda88aaa3769dde801db294c0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/363664


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>