math: fix typo and braino in my earlier commit

The bug number was a typo, and I forgot to switch the implementation
back to if statements after the change from Float64bits in the first
patchset back to branching.

if statements can currently be inlined, but switch cannot (#13071)

Change-Id: I81d0cf64bda69186c3d747a07047f6a694f8fa70
Reviewed-on: https://go-review.googlesource.com/16446Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/abs.go

@@ -10,13 +10,13 @@ package math
 //	Abs(±Inf) = +Inf
 //	Abs(NaN) = NaN
 func Abs(x float64) float64 {
-	// TODO: once golang.org/issue/13905 is fixed, change this to:
+	// TODO: once golang.org/issue/13095 is fixed, change this to:
 	// return Float64frombits(Float64bits(x) &^ (1 << 63))
 	// But for now, this generates better code and can also be inlined:
-	switch {
-	case x < 0:
+	if x < 0 {
 		return -x
-	case x == 0:
+	}
+	if x == 0 {
 		return 0 // return correctly abs(-0)
 	}
 	return x