[SelectionDAG][GlobalISel] Move even divisor optimization for division by constant...

[SelectionDAG][GlobalISel] Move even divisor optimization for division by constant into UnsignedDivideUsingMagic implementation. NFC

I've added a bool to UnsignedDivideUsingMagic so we can continue
testing it in the unit test with and without this optimization in
the unit test.

This is a step towards supporting "uncooperative" odd divisors.
See https://ridiculousfish.com/blog/posts/labor-of-division-episode-iii.html

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D140924