[DependenceInfo] Compute WAR dependence info using ISL kills. NFC.
When reading code of Dependences::calculateDependences, I noticed that
WAR is computed specifically by buildWAR. Given ISL now
supports "kills" in approximate dataflow analysis, this patch takes
advantage of it.
This patch also cleans up a couple lines redundant codes.
Patch by bin.narwal <firstname.lastname@example.org>
Differential Revision: https://reviews.llvm.org/D66741