[PATCH] MachineBlockPlacement: Strengthen the source order bias when picking an exit block.

[Chandler Carruth]

  Chatted on IRC a bit about this. I'm still quite nervous about exactly what magic constant we should be using here.

  I suggested to instead put this magic constant into a flag, and even set that flag up to give the old behavior. Then several of us can run a bunch of benchmarks across a spectrum of flag values and see if there is a clear peak.

  I'm happy for you to commit with a flag and preserving the old behavior (for now) with the test case and FIXMEs to find some way to lay out this loop The Right Way. Then we can evaluate the specific value post-commit and figure out if there is a value that is broadly good and fixes this (important) test case.

http://llvm-reviews.chandlerc.com/D2202