[NFC] GPU ukernels cleanups

[bjacob]

1. Rename UKernelSpec to UKernelConfig. I was grappling for the right word, but now that it's part of LoweringConfig, it's clearer.
2. Drop unused KernelConfig case for ukernel ops. The lowering to ukernel ops happens after KernelConfig.
3. To stringify types, instead of using a stringstream, we can actually just use llvm::formatv.
4. Reorganize LLVMGPUSelectUKernels.cpp to make it easier to add logic for other ukernels.