[llvm] [AMDGPU][TTI] Refine gfx9 packed FP32 SLP costs for pair formation and shuffles (PR #208572)

[Akash Dutta via llvm-commits]

akadutta wrote:

> The cost model test changes appear incidental. We should have a dedicated shuffle cost model test.
> 
> Gating this on the specific element type doesn't sound right. You don't know just from the type the use context. This also is implicitly assuming the legal 2 element case, directly folding into the small set of pk instructions.
> 
> Looking at the 2 element shuffle in isolation, 3 of them are a cheap, and it's only the high-to-low shuffle that's implemented with v_pk_mov_b32

Tests updated and 2 element shuffle cost modeling updated. 

https://github.com/llvm/llvm-project/pull/208572