[InstCombine] Handle scalable geps in EmitGEPOffset (#71565)

This adds scalable handling for scalable vectors in emitGEPOffset. This
was noticed in some tests that Biplob was creating, so might be unlikely
to come up much in practice. I've attempted to add test coverage for
various places EmitGEPOffset is called. The vscale intrinsics will
currently emit multiple copies, relying on later CSE to combine them.