math: use Abs rather than if x < 0 { x = -x }

This is the benchmark result base on darwin with amd64 architecture:

name     old time/op  new time/op  delta
Cos      10.2ns ± 2%  10.3ns ± 3%  +1.18%  (p=0.032 n=10+10)
Cosh     25.3ns ± 3%  24.6ns ± 2%  -3.00%  (p=0.000 n=10+10)
Hypot    6.40ns ± 2%  6.19ns ± 3%  -3.36%  (p=0.000 n=10+10)
HypotGo  7.16ns ± 3%  6.54ns ± 2%  -8.66%  (p=0.000 n=10+10)
J0       66.0ns ± 2%  63.7ns ± 1%  -3.42%  (p=0.000 n=9+10)

Fixes #21812

Change-Id: I2b88fbdfc250cd548f8f08b44ce2eb172dcacf43
Reviewed-on: https://go-review.googlesource.com/84437Reviewed-by: Giovanni Bajo <rasky@develer.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/hypot.go

@@ -26,12 +26,7 @@ func hypot(p, q float64) float64 {
 	case IsNaN(p) || IsNaN(q):
 		return NaN()
 	}
-	if p < 0 {
-		p = -p
-	}
-	if q < 0 {
-		q = -q
-	}
+	p, q = Abs(p), Abs(q)
 	if p < q {
 		p, q = q, p
 	}

src/math/j0.go

@@ -99,9 +99,7 @@ func J0(x float64) float64 {
 		return 1
 	}
 
-	if x < 0 {
-		x = -x
-	}
+	x = Abs(x)
 	if x >= 2 {
 		s, c := Sincos(x)
 		ss := s - c

src/math/sin.go

@@ -131,9 +131,7 @@ func cos(x float64) float64 {
 
 	// make argument positive
 	sign := false
-	if x < 0 {
-		x = -x
-	}
+	x = Abs(x)
 
 	j := int64(x * M4PI) // integer part of x/(Pi/4), as integer for tests on the phase angle
 	y := float64(j)      // integer part of x/(Pi/4), as float

src/math/sinh.go

@@ -71,9 +71,7 @@ func sinh(x float64) float64 {
 func Cosh(x float64) float64
 
 func cosh(x float64) float64 {
-	if x < 0 {
-		x = -x
-	}
+	x = Abs(x)
 	if x > 21 {
 		return Exp(x) / 2
 	}