FLD1 pushes +1.0 to the 387 register stack, and FLDZ pushes +0.0
to the 387 regiser stack.

They can be used to simplify MOVSSconst/MOVSDconst when the
constant is +0.0, -0.0, +1.0, -1.0.

The size of the go executable reduces about 62KB and the total size
of pkg/linux_386 reduces about 7KB with this optimization.

Change-Id: Icc8213b58262e0024a277cf1103812a17dd4b05e
Reviewed-on: https://go-review.googlesource.com/119635
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>