We insert extensions when lowering comparisons and they were
blocking constant folding.

Change-Id: I804bbf91c7606612ffe921a90853844a57e55955
Reviewed-on: https://go-review.googlesource.com/30541Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>