cmd/compile: fix mis-compilation when switching over channels
CL 418101 changes Unified IR writer to force mixed tag/case to have
common type, emitting the implicit conversion if any of the case values
are not assignable to the tag value's type.
However, the Go spec definition of equality is non-transitive for
channels stored in interfaces, causing incorrect behavior with channel
values comparison.
To fix it, don't emit the implicit conversions if tag type is channel.
Fixes #67190
Change-Id: I9a29d9ce3c7978f0689e9502ba6f15660c763d16
Reviewed-on: https://go-review.googlesource.com/c/go/+/594575
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Than McIntosh <thanm@google.com>