[LoopInterchange][PR56275] Fix legality with negative dependence vectors

[LoopInterchange][PR56275] Fix legality with negative dependence vectors

This is the 2nd patch of the two-patch series (D130188, D130189) that
fix PR56275 (https://github.com/llvm/llvm-project/issues/56275) which
is a missed opportunity for loop interchange.

As follow-up on the dependence analysis (DA) patch D130188, this patch
normalizes DA results in loop interchange, such that negative dependence
vectors queried by loop interchange are reversed to be non-negative.

Now all tests in PR56275 can get interchanged. Those tests are added
in lit test as `pr56275.ll`.

Reviewed By: kawashima-fj, bmahjour, Meinersbur, #loopoptwg

Differential Revision: https://reviews.llvm.org/D130189