math/big: fix ModSqrt optimized path for x = z

math/big: fix ModSqrt optimized path for x = z

name                   old time/op  new time/op  delta
ModSqrt224_3Mod4-4      153µs ± 2%   154µs ± 1%   ~     (p=0.548 n=5+5)
ModSqrt5430_3Mod4-4     776ms ± 2%   791ms ± 2%   ~     (p=0.222 n=5+5)

Fixes #22265

Change-Id: If233542716e04341990a45a1c2b7118da6d233f7
Reviewed-on: https://go-review.googlesource.com/70832
Run-TryBot: Filippo Valsorda <hi@filippo.io>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>