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

[Florian Hahn via llvm-commits]

fhahn wrote:

If the only planned use of this is to avoid printing `TC: 0` in LV, would it be possible  to simply update the code in LV to check that the return value is != 0 before printing? IIUC 0 is never a valid return value for `getSmallConstantTripCount` and it should be fairly clear from the documentation.

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