Support poison values

[int3]

The test added in triton-lang#4896 fails right now with an "LLVM Translation failed" error. The fix should be pretty simple; we just need to add a call to mlir::ub::populateUBToLLVMConversionPatterns like the nvidia/amd backends have done. However, since we use a somewhat different pass organization structure, we don't have a generic pattern conversion pass to put this into. I tried adding the conversion in FuncOpToLLVM, and while translation succeeds, the poison value is still missing from the llir output; I think it's getting DCE'd away. Not sure if this is a pass ordering issue, or if the test should just be made more flexible...

This doesn't seem super critical so I'm just filing an issue for now

[ienkovich]

We can add a proper pass to our pipeline similar to other upstream dialects lowering passes. It can be created using createUBToLLVMConversionPass.