[BasicAA] Remove unnecessary known size requirement

The size requirement on V2 was present because it was not clear
whether an unknown size would allow an access before the start of
V2, which could then overlap. This is clarified since D91649: In
this part of BasicAA, all accesses can occur only after the base
pointer, even if they have unknown size.

This makes the positive and negative offset cases symmetric.

Differential Revision: https://reviews.llvm.org/D91482