[PATCH] don't unroll loops with zero trip count; fix for PR21409

[Sanjay Patel]

Hi hfinkel, atrick, majnemer,

In PR21409 ( http://llvm.org/bugs/show_bug.cgi?id=21409 ), the loop unroller tries to transform a loop with a backedge count of i32 4294967295. Adding 1 to that causes an overflow of the SCEV value for the trip count. This patch adds a check to prevent unrolling that type of loop.

http://reviews.llvm.org/D6200

Files:
  lib/Transforms/Utils/LoopUnrollRuntime.cpp
  test/Transforms/LoopUnroll/runtime-zero-tripcount.ll
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