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

[David Sherwood via llvm-commits]

david-arm wrote:

I've added comments explaining why it's ok to use the symbolic max variant. I also fixed rename a variable in createInitialVPlan that was shadowing a variable of the same name in the VPlan class.

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