[PATCH] Enable unrolling of multi-exit loops

[Mark Heffernan]

Hi atrick, jingyue,

This patch de-pessimizes the calculation of loop trip counts in ScalarEvolution in the presence of multiple exits.  Previously all loops exits had to have identical counts for a loop trip count to be considered computable.  This pessimization was implemented by calling getBackedgeTakenCount(L) rather than getExitCount(L, ExitingBlock) inside of ScalarEvolution::
getSmallConstantTripCount() (see the FIXME in the comments of that function).  The pessimization was added to fix a corner case involving undefined behavior (pr/16130).  This patch more precisely handles the undefined behavior case allowing the pessimization to be removed.

http://reviews.llvm.org/D5550

Files:
  include/llvm/Analysis/ScalarEvolution.h
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/trip-count-pow2.ll
  test/Transforms/LoopUnroll/scevunroll.ll
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