Some improvements related to the computation of isReachable.

Some improvements related to the computation of isReachable.
This fixes Bugzilla #1835 (http://llvm.org/bugs/show_bug.cgi?id=1835).
This patched is reviewed by Tanya and Dan. Dan tested and approved it.

The reason for the bad performance of the old algorithm is that it is very naive and scans every
time all nodes of the DAG in the worst case.

This patch introduces  a new algorithm based on the paper "Online algorithms
for maintaining the topological order of a directed acyclic graph" by
David J.Pearce and Paul H.J.Kelly. This is the MNR algorithm. It has a
linear time worst-case and performs much better in most situations.

The paper can be found here:
http://fano.ics.uci.edu/cites/Document/Online-algorithms-for-maintaining-the-topological-order-of-a-directed-acyclic-graph.html

The main idea of the new algorithm is to compute the topological ordering of the SNodes in the
DAG and to maintain it even after DAG modifications. The topological ordering allows for very fast
node reachability checks.

Tests on very big  input files with tens of thousands of instructions in a BB indicate huge
speed-ups (up to 10x compilation time improvement) compared to the old version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@48817 91177308-0d34-0410-b5e6-96231b3b80d8