[llvm] [SPIRV] Handle unknown intrinsics (PR #166284)

[Nathan Gauër via llvm-commits]

https://github.com/Keenuts commented:

Right, thanks for the comment. So if I understand, if tomorrow I implement a proper lowering for `llvm.readcyclecounter()` in SPIR-V, this won't break you, just go through another path right?

I think this should be OK, but could you add a comment in the test to explain that if someone implements proper lowering to `llvm.readcyclecounter`, then they should just replace the intrinsic with another unsupported one in the test as this isn't breaking somebody?

https://github.com/llvm/llvm-project/pull/166284