cmd/compile: deal with comparable embedded in a constraint

Ignore an embedded type in an interface which is the predeclared
interface "comparable" (which currently can only be in a type
constraint), since the name doesn't resolve and the "comparable" type
doesn't have any relevant methods (for the purposes of the compiler).

Added new test case graph.go that needs this fix.

Change-Id: I2443d2c3dfeb9d0a78aaaaf91a2808ae2759d247
Reviewed-on: https://go-review.googlesource.com/c/go/+/301831


Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>