[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

llvm-svn: 258184

0 files changed, 0 insertions, 0 deletions