[llvm] [LLVM][SCEV] Look through common vscale multiplicand when simplifying compares. (PR #141798)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

I've simplified the PR and removed the ll tests that proved convoluted due to the awkwardness of verifying SCEV::SimplifyICmpOperands. I've instead implemented a unit-test to verify the new code in isolation and then there's the loop-vectorisation tests to show the effect.

https://github.com/llvm/llvm-project/pull/141798