[llvm] [SLP][NFC] Add non-power-of-2 buildvector test (PR #206332)

[Harrison Hao via llvm-commits]

harrisonGPU wrote:

Hi @alexey-bataev , could you please take a look this NFC PR :-) And then I will rebase the https://github.com/llvm/llvm-project/pull/206259

https://github.com/llvm/llvm-project/pull/206332