[llvm] [LoopVectorize] In LoopVectorize.cpp start using getSymbolicMaxBackedgeTakenCount (PR #108833)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm approved this pull request.

I think you might be taking the "create small patches" idea a bit too far but I suppose this change does ensure LoopVectorize is not relying on the more restricted behaviour of `getBackedgeTakenCount()`.

https://github.com/llvm/llvm-project/pull/108833