[dev.typeparams] cmd/compile: use r.hasTypeParams in typIdx

CL 329571 fold the checking has type params logic, but did not realize
that the instance in typIdx can be folded, too.

Change-Id: I4682af3779535af6a6e843972cada12ba1bae6ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/330389


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