compiler, vk: Support subgroup size of 4

Relax the assert and assign it an enum value

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30876>

src/vulkan/runtime/vk_pipeline.c

@@ -110,7 +110,7 @@ vk_get_subgroup_size(uint32_t spirv_version,
    uint32_t req_subgroup_size = get_required_subgroup_size(info_pNext);
    if (req_subgroup_size > 0) {
       assert(util_is_power_of_two_nonzero(req_subgroup_size));
-      assert(req_subgroup_size >= 8 && req_subgroup_size <= 128);
+      assert(req_subgroup_size >= 4 && req_subgroup_size <= 128);
       return req_subgroup_size;
    } else if (allow_varying || spirv_version >= 0x10600) {
       /* Starting with SPIR-V 1.6, varying subgroup size the default */