[llvm] [LoopVectorize] Add cost of generating tail-folding mask to the loop (PR #90191)

[David Sherwood via llvm-commits]

david-arm wrote:

When adding the cost of the lane mask to the loop in some cases we just decide it's not worth vectorising. Whilst this is probably the correct decision from a performance perspective, it does mean I had to add attributes so that we're still testing the code paths intended by the original tests.

https://github.com/llvm/llvm-project/pull/90191