[llvm] [NVPTX][NFC] Remove unneeded declarations in test (PR #101167)

[Hugh Delaney via llvm-commits]

hdelan wrote:

> Are these declarations tested elsewhere?
> 
> It's not clear to me that the fix is removing them, vs adding missing tests for them.

These declarations are used and the PTX lowering tested in this file. But they don't need to be declared in the test file since they are LLVM intrinsics. By contrast the bf16 `llvm.` declarations are needed since the bf16 funcs are not real LLVM intrinsics. Their lowerings are hoisted [here](https://github.com/llvm/llvm-project/blob/main/llvm/lib/IR/AutoUpgrade.cpp#L918)

https://github.com/llvm/llvm-project/pull/101167