math/big: handle alias of cofactor inputs in GCD

If the variables passed in to the cofactor arguments of GCD (x, y)
aliased the input arguments (a, b), the previous implementation would
result in incorrect results for y.  This change reorganizes the calculation
so that the only case that need to be handled is when y aliases b, which
can be handled with a simple check.

Tests were added for all of the alias cases for input arguments and and
and irrelevant test case for a previous binary GCD calculation was dropped.

Fixes #30217

Change-Id: Ibe6137f09b3e1ae3c29e3c97aba85b67f33dc169
Reviewed-on: https://go-review.googlesource.com/c/162517


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>