math: optimize sinh and cosh

Improve performance by reducing unnecessary function calls

Benchmarks:

Tme    old time/op  new time/op  delta
Cosh-8   229ns ± 0%   138ns ± 0%  -39.74%  (p=0.008 n=5+5)
Sinh-8   231ns ± 0%   139ns ± 0%  -39.83%  (p=0.008 n=5+5)

Change-Id: Icab5485849bbfaafca8429d06b67c558101f4f3c
Reviewed-on: https://go-review.googlesource.com/85477
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/sinh.go

@@ -45,10 +45,11 @@ func sinh(x float64) float64 {
 	var temp float64
 	switch true {
 	case x > 21:
-		temp = Exp(x) / 2
+		temp = Exp(x) * 0.5
 
 	case x > 0.5:
-		temp = (Exp(x) - Exp(-x)) / 2
+		ex := Exp(x)
+		temp = (ex - 1/ex) * 0.5
 
 	default:
 		sq := x * x
@@ -73,7 +74,8 @@ func Cosh(x float64) float64
 func cosh(x float64) float64 {
 	x = Abs(x)
 	if x > 21 {
-		return Exp(x) / 2
+		return Exp(x) * 0.5
 	}
-	return (Exp(x) + Exp(-x)) / 2
+	ex := Exp(x)
+	return (ex + 1/ex) * 0.5
 }