cmd/6g, cmd/8g, liblink: improve handling of float constants

* Enable basic constant propagation for floats.
  The constant propagation is still not as aggressive as it could be.
* Implement MOVSS $(0), Xx and MOVSD $(0), Xx as XORPS Xx, Xx.

Sample code:

func f32() float32 {
	var f float32
	return f
}

func f64() float64 {
	var f float64
	return f
}

Before:

"".f32 t=1 size=32 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:3)	TEXT	"".f32+0(SB),4,$0-8
	0x0000 00000 (demo.go:3)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:3)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:3)	MOVSS	$f32.00000000+0(SB),X0
	0x0008 00008 (demo.go:4)	MOVSS	$f32.00000000+0(SB),X0
	0x0010 00016 (demo.go:5)	MOVSS	X0,"".~r0+8(FP)
	0x0016 00022 (demo.go:5)	RET	,
"".f64 t=1 size=32 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:8)	TEXT	"".f64+0(SB),4,$0-8
	0x0000 00000 (demo.go:8)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:8)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:8)	MOVSD	$f64.0000000000000000+0(SB),X0
	0x0008 00008 (demo.go:9)	MOVSD	$f64.0000000000000000+0(SB),X0
	0x0010 00016 (demo.go:10)	MOVSD	X0,"".~r0+8(FP)
	0x0016 00022 (demo.go:10)	RET	,

After:

"".f32 t=1 size=16 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:3)	TEXT	"".f32+0(SB),4,$0-8
	0x0000 00000 (demo.go:3)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:3)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:3)	XORPS	X0,X0
	0x0003 00003 (demo.go:5)	MOVSS	X0,"".~r0+8(FP)
	0x0009 00009 (demo.go:5)	RET	,
"".f64 t=1 size=16 value=0 args=0x8 locals=0x0
	0x0000 00000 (demo.go:8)	TEXT	"".f64+0(SB),4,$0-8
	0x0000 00000 (demo.go:8)	FUNCDATA	$0,gclocals·a7a3692b8e27e823add69ec4239ba55f+0(SB)
	0x0000 00000 (demo.go:8)	FUNCDATA	$1,gclocals·3280bececceccd33cb74587feedb1f9f+0(SB)
	0x0000 00000 (demo.go:8)	XORPS	X0,X0
	0x0003 00003 (demo.go:10)	MOVSD	X0,"".~r0+8(FP)
	0x0009 00009 (demo.go:10)	RET	,

Change-Id: Ie9eb65e324af4f664153d0a7cd22bb16b0fba16d