Allow traces to enter nested loops.
Allow traces to enter nested loops.

This lets traces include the final iteration of a nested loop above the
center block, and the first iteration of a nested loop below the center
block.

We still don't allow traces to contain backedges, and traces are
truncated where they would leave a loop, as seen from the center block.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@161003 91177308-0d34-0410-b5e6-96231b3b80d8

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