[LAA] Factor out logic to compute dependence distance. (NFCI)

This patch refactors the logic to compute the dependence distance,
stride, size and write info to a separate function. This limits the
scope of various A* and B* variables, which in turn makes it easier to
reason about their uses.

In particular this makes it more explicit why dropping the various
std::swaps as done in https://github.com/llvm/llvm-project/pull/70819/
is valid.