Previously, given two Nodes n1 and n2 of different non-PAUTO classes
(e.g., PPARAM and PPARAMOUT), cmpstackvarlt(n1, n2) and
cmpstackvarlt(n2, n1) both returned true, which is nonsense.

This doesn't seem to cause any visible miscompilation problems, but
notably fixing it does cause toolstash/buildall to fail.

Change-Id: I33b2c66e902c5eced875d8fbf18b7cfdc81e8aed
Reviewed-on: https://go-review.googlesource.com/19778
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>