[Loads] Support dereferenceable assumption with variable size. (#128436)
Update isDereferenceableAndAlignedPointer to make use of dereferenceable
assumptions with variable sizes via SCEV.
To do so, factor out the logic to check via an assumption to a helper,
and use SE to check if the access size is less than the dereferenceable
size.
PR: https://github.com/llvm/llvm-project/pull/128436