[llvm] [SCEV] Check whether the start is non-zero in `ScalarEvolution::howFarToZero` (PR #131522)

[Philip Reames via llvm-commits]

preames wrote:

Thank you for finding and fixing this one.  I'm a bit shocked I managed to miss that condition and it took this long to find.  Initially, I'd suspected we had some kind of partial guard somewhere, but after some quick digging, I couldn't find one.  So, this appears to have just been wrong for this case from the start.

https://github.com/llvm/llvm-project/pull/131522