[SPIR-V] Implement correct zeroinitializer for extension types in SPIR-V Backend (#93607)

This PR implements correct zeroinitializer for extension types in SPIR-V
Backend.

Previous version has just created 0 of 32/64 integer type (depending on
target machine word size), that caused re-use and type re-write of the
corresponding integer constant 0 with a potential crash on wrong usage
of the constant (i.e., 0 of integer type expected but extension type
found). E.g., the following code would crash without the PR:

```
  %r1 = icmp ne i64 %_arg_i, 0
  %e1 = tail call spir_func target("spirv.Event") @__spirv_GroupAsyncCopy(i32 2, ptr addrspace(3) %_arg_local, ptr addrspace(1) %_arg_ptr, i64 1, i64 1, target("spirv.Event") zeroinitializer)
```

because 0 in icmp would eventually be of `Event` type.