[GlobalISel] Handle non-multiples of the base type in narrowScalarAddSub

When narrowing G_ADD and G_SUB, handle types that aren't a multiple of
the type we're narrowing to. This allows us to handle types like s96
on 64 bit targets.

Note that the test here has a couple of dead instructions because of
the way the setup legalizes. I wasn't able to come up with a way to
write this test that avoids that easily.

Differential Revision: https://reviews.llvm.org/D97811