[PATCH] [LoopUnrollRuntime] Avoid high-cost trip count computation.

[Sanjoy Das]

I've added `AllowExpensiveTripCount` to `UnrollPreferences`.  I don't
have a good understanding for when `AllowExpensiveTripCount` should be
true, so I've left it `false` always.

- address Hal's review: add UP.AllowExpensiveTripCount, off by default


http://reviews.llvm.org/D8994

Files:
  include/llvm/Analysis/TargetTransformInfo.h
  include/llvm/Transforms/Utils/UnrollLoop.h
  lib/Transforms/Scalar/LoopUnrollPass.cpp
  lib/Transforms/Utils/LoopUnroll.cpp
  lib/Transforms/Utils/LoopUnrollRuntime.cpp
  test/Transforms/LoopUnroll/high-cost-trip-count-computation.ll
  test/Transforms/LoopUnroll/runtime-loop.ll
  test/Transforms/LoopUnroll/runtime-loop4.ll

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