Extend ScalarEvolution's getBackedgeTakenCount to be able to

Extend ScalarEvolution's getBackedgeTakenCount to be able to
compute an upper-bound value for the trip count, in addition to
the actual trip count. Use this to allow getZeroExtendExpr and
getSignExtendExpr to fold casts in more cases.

This may eventually morph into a more general value-range
analysis capability; there are certainly plenty of places where
more complete value-range information would allow more folding.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70509 91177308-0d34-0410-b5e6-96231b3b80d8