[llvm] [SPIR-V] Fix OpGroupAsyncCopy null event substitution (PR #212754)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

> Looks good to me.
> 
> If the event is not null we still generate invalid code right? Do we have a test for that?

Yes, that's right, that is existing problem. I haven't found any tests for that, actually 

https://github.com/llvm/llvm-project/pull/212754