[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 <bin.narwal@gmail.com>

Differential Revision: https://reviews.llvm.org/D66741

llvm-svn: 370396