[llvm] [LV]: Improve accuracy of calculating remaining iterations of MainLoopVF (PR #156723)

[Hassnaa Hamdi via llvm-commits]

================

----------------
hassnaaHamdi wrote:

I locally did that and eventually compared these changes to the final changes I found them same. I think I already did that initially. 

https://github.com/llvm/llvm-project/pull/156723