cmd/compile: convert some Phis into And8.
See discussion at [1]. True value must have a fixed non-zero
representation meaning that a && b can be implemented as a & b.
[1] https://groups.google.com/forum/#!topic/golang-dev/xV0vPuFP9Vg
This change helps with m := a && b, but it's more common to see
if a && b { do something } which is not handled.
Change-Id: Ib6f9ff898a0a8c05d12466e2464e4fe781035394
Reviewed-on: https://go-review.googlesource.com/22313
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>