[llvm] [SPIR-V] Simplify addrspacecast for null in composite constant preprocessing (PR #192030)

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

> The fix is LGTM, but I wonder if SPIR-V backend (today) can handle a sample like this:
> 
> ```
> define spir_kernel void @bar(ptr addrspace(1) %arg) {
> entry:
>   %expr = icmp eq ptr addrspace(1) addrspacecast (ptr null to ptr addrspace(1)), %arg
>   ret void
> }
> ```
> 
> If no - the fix in this PR should be generalized.

Covered this scenario as well, thanks

https://github.com/llvm/llvm-project/pull/192030