[llvm] [LV] Only check isVectorizableEarlyExitLoop with multiple exits. (PR #121994)

[Philip Reames via llvm-commits]

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

LGTM

It's worth noting that the style of logic used by the early exit reasoning could in theory vectorize some single exit loops which have a countable upper bound, but not an exact countable exit.  Said differently, we could execute additional iterations when safe to do so.  The current code doesn't do so, but we could reasonable extend it (in some future patch).  

https://github.com/llvm/llvm-project/pull/121994