[PATCH] Partial fix for bug 22589

[Sanjoy Das]

Hi mzolotukhin, hfinkel, spatel,

This change tries to solve the problem solved by revision 222451 in a way that does not require scalarizing the entire iteration space in case of overflow when computing the trip count of a runtime-unrolled loop.  It also gets rid of the backedge check in the prologue loop and the extra check for overflowing trip-count.

I'm not familiar with this area, so would appreciate a close review.

http://reviews.llvm.org/D7715

Files:
  lib/Transforms/Utils/LoopUnrollRuntime.cpp
  test/Transforms/LoopUnroll/runtime-loop.ll
  test/Transforms/LoopUnroll/runtime-loop1.ll
  test/Transforms/LoopUnroll/tripcount-overflow.ll

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