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

name     old time/op  new time/op  delta
PowInt   55.7ns ± 1%  53.4ns ± 2%  -4.15%  (p=0.000 n=9+9)
PowFrac   133ns ± 1%   133ns ± 2%    ~     (p=0.587 n=8+9)

Change-Id: Ica0f4c2cbd554f2195c6d1762ed26742ff8e3924
Reviewed-on: https://go-review.googlesource.com/c/85375Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/pow.go

@@ -83,13 +83,7 @@ func pow(x, y float64) float64 {
 		return 1 / Sqrt(x)
 	}
 
-	absy := y
-	flip := false
-	if absy < 0 {
-		absy = -absy
-		flip = true
-	}
-	yi, yf := Modf(absy)
+	yi, yf := Modf(Abs(y))
 	if yf != 0 && x < 0 {
 		return NaN()
 	}
@@ -147,9 +141,9 @@ func pow(x, y float64) float64 {
 	}
 
 	// ans = a1*2**ae
-	// if flip { ans = 1 / ans }
+	// if y < 0 { ans = 1 / ans }
 	// but in the opposite order
-	if flip {
+	if y < 0 {
 		a1 = 1 / a1
 		ae = -ae
 	}