math/big: recognize squaring for Floats

Updates #13745

Recognize z.Mul(x, x) as squaring for Floats and use
the internal z.sqr(x) method for nat on the mantissa.

Change-Id: I0f792157bad93a13cae1aecc4c10bd20c6397693
Reviewed-on: https://go-review.googlesource.com/56774Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/big/float.go

@@ -1311,8 +1311,11 @@ func (z *Float) umul(x, y *Float) {
 	// TODO(gri) Optimize this for the common case.
 
 	e := int64(x.exp) + int64(y.exp)
-	z.mant = z.mant.mul(x.mant, y.mant)
-
+	if x == y {
+		z.mant = z.mant.sqr(x.mant)
+	} else {
+		z.mant = z.mant.mul(x.mant, y.mant)
+	}
 	z.setExpAndRound(e-fnorm(z.mant), 0)
 }