[llvm] [LV][AArch64] Prefer Fixed over Scalable if cost-model is equal (Neoverse V2) (PR #95819)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> @paulwalker-arm , @david-arm : I think a very fair question is why we should be generating SVE code for this type of kernels? Can you explain the benefit?

The general answer is the cost model says there's no reason not to use scalable vectors, and with the potential for them to be bigger than their fixed length counterparts there's scope for more performance beyond the assumed minimum.

What worries me is that you're not suggesting there's a subtle performance difference but in fact the difference is pretty large.  I just think this should be captured as ether a more accurate cost model or implementation specific code generation decisions (e.g. `UseScalarIncVL`).

https://github.com/llvm/llvm-project/pull/95819