[AA] Improve the BasicAA analysis capability

According https://discourse.llvm.org/t/memoryssa-does-the-accessedbetween-support-scalable-vector-pointer/65052,
scalable vector support in BasicAA is currently essentially limited,
and should be improved effectively for a constant offset GEP if the scalable index is zero, eg:
  getelementptr <vscale x 4 x i32>, ptr %p, i64 0, i64 %i

Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D133567