Enable GL_KHR_shader_subgroup properly

(and GL_NV_shader_subgroup_partitioned) based on GL/ES version
instead of predicating it on vulkan SPV generation

Also add AST testing.
The glsl.450.subgroup* files are largely the same as the spv.subgroup*
The glsl.es320.subgroup* files are the same as the 450 versions, but modified to be ES compatible.

Test/glsl.450.subgroup.frag

@@ -0,0 +1,7 @@
+#version 450
+#extension GL_KHR_shader_subgroup_basic: enable
+layout(location = 0) out uvec4 data;
+void main (void)
+{
+  data = uvec4(gl_SubgroupSize, gl_SubgroupInvocationID, 0, 0);
+}