[LAA] Use type sizes when determining dependence.

In the isDependence function the code does not try hard enough
to determine the dependence between types. If the types are
different it simply gives up, whereas in fact what we really
care about are the type sizes. I've changed the code to compare
sizes instead of types.

Reviewed By: fhahn, sdesmalen

Differential Revision: https://reviews.llvm.org/D108763