Eliminate reachability matrix. It has to be calculated before any instruction

Eliminate reachability matrix. It has to be calculated before any instruction
selection is done. That's rather expensive especially in situations where it
isn't really needed.
Move back to a searching the predecessors, but make use of topological order
to trim the search space.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29559 91177308-0d34-0410-b5e6-96231b3b80d8