cmd/compile: fix select statement evaluation order corner case

The Go spec requires that select case clauses be evaluated in order,
which is stricter than normal ordering semantics. cmd/compile handled
this correctly for send clauses, but was not correctly handling
receive clauses that involved bare variable references.

Discovered with @cuonglm.

Fixes #43111.

Change-Id: Iec93b6514dd771875b084ba49c15d7f4531b4a6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/277132


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