Materialize float constant 0 from integer zero register, instead
of loading from constant pool.

Change-Id: Ie4728895b9d617bec2a29d15729c0efaa10eedbb
Reviewed-on: https://go-review.googlesource.com/32109
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>