[SYCL][InvokeSimd] Allow callables to return uniform

[sarnex]

The spec states that returning a uniform object is allowed:

"Return values of type sycl::ext::oneapi::experimental::uniform are not anyhow converted, and broadcast to each work-item; every work-item in the sub-group receives the same value. NOTE: sycl::ext::oneapi::experimental::uniform return type is the way to return a uniform value of simd or simd_mask type."

Update the compile-time error checking and ESIMD verifier to allow this.

[turinevgeny]

LGTM.

[v-klochkov]

Using 'uniform' in ESIMD context does not seem needed at all (IMO).

int/*uniform_return*/  callee(simd< int, 8 > vec, int uniform_param) SYCL_ESIMD_FUNCTION" 

is enough to express the uniform-ness of the returned value and the second argument.

I suppose this patch is needed because of this invoke_simd SPEC statements that implicitly tells using uniform in ESIMD context is Ok, right?

Return values of type T are converted to sycl::ext::oneapi::experimental::uniform< T >, and broadcast to each work-item; every work-item in the sub-group receives the same value.

[sarnex]

Yes, this is specifically needed because the invoke_simd spec defines uniform as a valid return type for ESIMD functions that are called through invoke_simd. We don't have a way to check at compile time if an ESIMD function is called by invoke_simd or not because it is/can be through a function pointer, so we just allow it in all cases.