Simplify computation of reduction dependences

This simplifies the construction of the input for the reduction dependence
computation and at the same time removes an assumption that expects the schedule
to be of 2D + 1 form (the odd dimensions giving textual order, the even
dimensions the loop iterations).

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@223621 91177308-0d34-0410-b5e6-96231b3b80d8