Deal with irreducible control flow when building traces.

Deal with irreducible control flow when building traces.

We filter out MachineLoop back-edges during the trace-building PO
traversals, but it is possible to have CFG cycles that aren't natural
loops, and MachineLoopInfo doesn't include such cycles.

Use a standard visited set to detect such CFG cycles, and completely
ignore them when picking traces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161532 91177308-0d34-0410-b5e6-96231b3b80d8