go/types: fix unsymmetric test when typechecking comparisons

The existing code assumed that comparability and orderedness
was implied for the 2nd operand if the 1st operand satisfied
these predicates.

Fixes #28164.

Change-Id: I61d4e5eedb3297731a20a14acb3645d11b36fcc5
Reviewed-on: https://go-review.googlesource.com/c/143277


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>