cmd/compile: lazily create true and false Values in shortcircuit

cmd/compile: lazily create true and false Values in shortcircuit

It is mildly wasteful to always create values
that must sometimes then be dead code eliminated.
Given that it is very easy to avoid, do so.

Noticed when examining a package with thousands
of generated wrappers, each of which uses
only a handful of Values to compile.

Change-Id: If02eb4aa786dfa20f7aa43e8d729dad8b3db2786
Reviewed-on: https://go-review.googlesource.com/41502
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>