Some improvements related to the computation of heights, depths of SUnits.

Some improvements related to the computation of heights, depths of SUnits.

The basic idea is that all these algorithms are computing the longest paths from the root node or to the exit node. Therefore the existing implementation that uses and iterative and potentially
exponential algorithm was changed to a well-known graph algorithm based on dynamic programming. It has a linear run-time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47884 91177308-0d34-0410-b5e6-96231b3b80d8