[llvm] [NFC][NVPTX] Cleanup getPreferredVectorAction() (PR #114115)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

> What are the existing tests that verify the behavior is unchanged here?

Any test that uses `v2*16` types. 
- `bf16-instructions.ll`
- `bf16x2-instructions.ll`
- `bf16x2-instructions-approx.ll`
- `convert-sm80.ll`
- `bswap.ll`
- ...

https://github.com/llvm/llvm-project/pull/114115