Define buildScheduleRec on RegionNodes and pull out the tree traversal [NFC]

This change clarifies that for Not-NonAffine-SubRegions we actually iterate over
the subnodes and for both NonAffine-SubRegions and BasicBlocks, we perform the
schedule construction. As a result, the tree traversal becomes trivial, the
special case for a scop consisting just of a single non-affine region
disappears and the indentation of the code is reduced.

No functional change intended.

llvm-svn: 256940