support subgroup broadcast for pointers

[psteinbrecher]

Subgroup broadcast is not supported for pointers. This could potentially be used as a compiler hint that pointers are uniform within a subgroup.

void foo( float *ptr, intel::sub_group sg ) {

    float *ptr_uniform = cl::sycl::intel::broadcast<intel::sub_group,float*>( sg, ptr );

   // kernel that performs subgroup loads using ptr_uniform
}