Don't attempt to analyze the decomposed GEP for scalable type.
GEP index scale is not compile-time constant for scalable type.
Be conservative, return MayAlias.
Explicitly call TypeSize::getFixedSize() to assert on places where
scalable type doesn't make sense.
Add unit tests to check functionality of -basicaa for scalable type.
This patch is needed for D76944.
I think this still doesn't completely block off all the relevant cases.
DecomposeGEPExpression is recursive, so there are can be multiple levels of GEPs. The GEP over the scalable vector may not be GEP1; it could be GEP1->getPointerOperand(), or something like that.