[llvm] [LV] Use SCEV to check if IV overflow check is known (PR #115705)

[Luke Lau via llvm-commits]

lukel97 wrote:

As a side note, on RISC-V vscale always has to be a power of 2 so I think we might always be able to elide the overflow check. But I'm not sure if we have any way of expressing that constraint on vscale currently.

https://github.com/llvm/llvm-project/pull/115705