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

[Hassnaa Hamdi via llvm-commits]

hassnaaHamdi wrote:

There is a failure because this file `Transforms/LoopVectorize/AArch64/sve-epilog-vscale-fixed.ll`  got affected and I didn't update it deliberately because the new change will hide the original motivation behind the test. So I will create a NFC for that test file to create a new test case that doesn't get affected by this patch.

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