[LoopCacheAnalysis]: Add support for negative stride

LoopCacheAnalysis currently assumes the loop will be iterated over in
a forward direction. This patch addresses the issue by using the
absolute value of the stride when iterating backwards.

Note: this patch will treat negative and positive array access the
same, resulting in the same cost being calculated for single and
bi-directional access patterns. This should be improved in a
subsequent patch.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D73064