Merging r258184:

Merging r258184:
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r258184 | sanjoy | 2016-01-19 12:53:51 -0800 (Tue, 19 Jan 2016) | 20 lines

[SCEV] Fix PR26207

In some cases, the max backedge taken count can be more conservative
than the exact backedge taken count (for instance, because
ScalarEvolution::getRange is not control-flow sensitive whereas
computeExitLimitFromICmp can be).  In these cases,
computeExitLimitFromCond (specifically the bit that deals with `and` and
`or` instructions) can create an ExitLimit instance with a
`SCEVCouldNotCompute` max backedge count expression, but a computable
exact backedge count expression.  This violates an implicit SCEV
assumption: a computable exact BE count should imply a computable max BE
count.

This change

 - Makes the above implicit invariant explicit by adding an assert to
   ExitLimit's constructor

 - Changes `computeExitLimitFromCond` to be more robust around
   conservative max backedge counts
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_38@258869 91177308-0d34-0410-b5e6-96231b3b80d8