cmd/compile/internal/types2: avoid duplicate errors for invalid bounds

Resolve a TODO from an earlier CL: we should only check type parameter
bounds once in collectTypeParams.

Change-Id: Icf6053ec359f8ac8143cf68ee2defd504f8f86e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/355069


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>