intel/compiler: Be more conservative about subgroup sizes in GL

The rules for gl_SubgroupSize in Vulkan require that it be a constant
that can be queried through the API.  However, all GL requires is that
it's a uniform.  Instead of always claiming that the subgroup size in
the shader is 32 in GL like we have to do for Vulkan, claim 8 for
geometry stages, the maximum for fragment shaders, and the actual size
for compute.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/intel/compiler/brw_compiler.h

@@ -203,9 +203,18 @@ struct brw_sampler_prog_key_data {
    float scale_factors[32];
 };
 
+/** An enum representing what kind of input gl_SubgroupSize is. */
+enum PACKED brw_subgroup_size_type
+{
+   BRW_SUBGROUP_SIZE_API_CONSTANT,  /**< Vulkan behavior */
+   BRW_SUBGROUP_SIZE_UNIFORM,       /**< OpenGL behavior */
+};
+
 struct brw_base_prog_key {
    unsigned program_string_id;
 
+   enum brw_subgroup_size_type subgroup_size_type;
+
    struct brw_sampler_prog_key_data tex;
 };