[llvm] [SCEV] Init BE vars to CouldNotCompute in howManyLT (NFC) (PR #222892)

[Antonio Frighetto via llvm-commits]

https://github.com/antoniofrighetto approved this pull request.

LG, with maybe the suggestion to preserve a rephrased version of the comment at the computeMaxBECountForLT() call site (else branch).

https://github.com/llvm/llvm-project/pull/222892