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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 llvm-svn: 258184
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