[llvm] [AArch64][SLP] Add NFC test cases for floating point reductions (PR #106507)

[Sushant Gokhale via llvm-commits]

sushgokh wrote:

> u need these to be based on the CPU? I assume your followup will alter the costs in some way?

Thought of adding a RUN line for Neoverse-v2 because
1. The throughput for 'faddp' instruction, generated for these reductions, has doubled from 2 to 4 for Neoverse-V1/V2 onwards
2. Want to avoid getting into a trap where you are not able to detect issues due to generalization for multiple cpus. Maybe if we try for a cost which is one size fits all, then we may be hiding some cost-modelling issues?

But  yeah, I haven't run the benchmarks yet with/without Neoverse-V2 option.

https://github.com/llvm/llvm-project/pull/106507