subgroups: fix Wsign-compare warnings (#1778)

The subgroup and workgroup sizes reported by clGetKernelSubGroupInfo
and clGetKernelWorkGroupInfo are of type `size_t`.  Avoid changing the
values to an `int` type as they are propagated through the tests and
then compared against `size_t` again.

Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

test_conformance/subgroups/subhelpers.cpp

@@ -206,7 +206,7 @@ void set_last_workgroup_params(int non_uniform_size, int &number_of_subgroups,
 }
 
 void fill_and_shuffle_safe_values(std::vector<cl_ulong> &safe_values,
-                                  int sb_size)
+                                  size_t sb_size)
 {
     // max product is 720, cl_half has enough precision for it
     const std::vector<cl_ulong> non_one_values{ 2, 3, 4, 5, 6 };