math/big: recognize squaring for Floats
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/56774
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,codestation/go,josharian/go,codestation/go,cloudflare/go,josharian/go,josharian/go,josharian/go,nawawi/go,Cofyc/go,akutz/go,Cofyc/go,akutz/go,polyverse-security/go,akutz/go,cloudflare/go,codestation/go,golang/go,polyverse-security/go,Cofyc/go,codestation/go,nawawi/go,nawawi/go,CAFxX/go,akutz/go,josharian/go,CAFxX/go,josharian/go,codestation/go,Cofyc/go,cloudflare/go,christopher-henderson/Go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,golang/go,golang/go,codestation/go,polyverse-security/go,polyverse-security/go,cloudflare/go,Cofyc/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,golang/go,CAFxX/go,akutz/go,Cofyc/go,CAFxX/go,codestation/go,Cofyc/go,Cofyc/go,akutz/go,polyverse-security/go,golang/go,akutz/go,CAFxX/go,codestation/go,josharian/go,josharian/go,polyverse-security/go,polyverse-security/go,nawawi/go,cloudflare/go,polyverse-security/go