[llvm] [DA] getBackedgeTakenCount in isKnownLessThan can return CouldNotCompute (PR #162495)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

It might be better to use `DependenceInfo::collectUpperBound` to obtain the BTC, as DA generally assumes the BTC to be loop-invariant with respect to the outermost loop when using it.

https://github.com/llvm/llvm-project/pull/162495