Factor out the gcd of the scalars when two `Product` summands are fused.

[bimalgaudel]

Eg.

• 1/2 AB + 1/4 AC = 1/4 A (2 B + C)
• - 1/2 AB - 1/4 AC = - 1/4 A (2 B + C)

In the second example, the gcd is factored out, and -1 is factored out afterward as a canonicalization step.