[LAA] Support backward dependences with non-constant distance. (#91525)

Following up to 933f4924, also update the code reasoning about
backwards dependences to support non-constant distances.

Update the code to use the signed minimum distance instead of a constant
distance

This means e checked the lower bound of the dependence distance and the
distance may be larger at runtime (and safe for vectorization). Whether
to classify it as Unknown or Backwards depends on the vector width and
LAA was updated to take TTI to get the maximum vector register width.

If the minimum dependence distance is larger than the max vector width,
we consider it as backwards-vectorizable. Otherwise we classify them as
Unknown, so we re-try with runtime checks.

PR: https://github.com/llvm/llvm-project/pull/91525