Fixes #23224

The previous Pow code had an optimization for
powers equal to ±0.5 that used Sqrt for
increased accuracy/speed.  This caused special
cases involving powers of ±0.5 to disagree with
the Pow spec.  This change places the Sqrt optimization
after all of the special case handling.

Change-Id: I6bf757f6248256b29cc21725a84e27705d855369
Reviewed-on: https://go-review.googlesource.com/85660Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>