SPV: Further refine OpenGL vs. Vulkan differences for SPIR-V.

Includes adding test cases to verify the differences.

Test/spv.specConst.vert

@@ -0,0 +1,8 @@
+#version 450
+
+layout(constant_id = 11) const int a = 8;
+
+void main()
+{
+    gl_Position = vec4(1.0) / a;
+}