[BasicAA] Improve scalable vector handling

Currently, DecomposeGEP() bails out on the whole decomposition if
it encounters a scalable GEP type anywhere. However, it is fine to
still analyze other GEPs that we look through before hitting the
scalable GEP. This does mean that the decomposed GEP base is no
longer required to be the same as the underlying object. However,
I don't believe this property is necessary for correctness anymore.

This allows us to compute slightly more precise aliasing results
for GEP chains containing scalable vectors, though my primary
interest here is simplifying the code.

Differential Revision: https://reviews.llvm.org/D110511