[Dependences] Compute reduction dependences on schedule tree [NFC]

This change gets rid of the need for zero padding, makes the reduction
computation code more similar to the normal dependence computation, and also
better documents what we do at the moment.

Making the dependence computation for reductions a little bit easier to
understand will hopefully help us to further reduce code duplication.

This reduces the time spent only in the reduction dependence pass from 260ms to
150ms for test/DependenceInfo/reduction_sequence.ll. This is a reduction of over
40% in dependence computation time.

This change was inspired by discussions with Michael Kruse, Utpal Bora,
Siddharth Bhat, and Johannes Doerfert. It can hopefully lay the base for further
cleanups of the reduction code.

llvm-svn: 295550

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