[llvm] SCEV: return std::nullopt for invalid TC (NFC) (PR #94162)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Also, just to double check, is that `TC: 0` print really a bug and not intentional? I would have thought it is relevant debug information that the trip count could not be computed.

If it's desired, we should print "Could not compute trip count", as opposed to a cryptic "Found TC: 0".

https://github.com/llvm/llvm-project/pull/94162