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

Includes adding test cases to verify the differences.

Test/baseResults/spv.glFragColor.frag.out

@@ -0,0 +1,30 @@
+spv.glFragColor.frag
+
+Linked fragment stage:
+
+
+// Module Version 10000
+// Generated by (magic number): 80001
+// Id's are bound by 12
+
+                              Capability Shader
+               1:             ExtInstImport  "GLSL.std.450"
+                              MemoryModel Logical GLSL450
+                              EntryPoint Fragment 4  "main" 9
+                              ExecutionMode 4 OriginLowerLeft
+                              Source GLSL 330
+                              Name 4  "main"
+                              Name 9  "gl_FragColor"
+               2:             TypeVoid
+               3:             TypeFunction 2
+               6:             TypeFloat 32
+               7:             TypeVector 6(float) 4
+               8:             TypePointer Output 7(fvec4)
+ 9(gl_FragColor):      8(ptr) Variable Output
+              10:    6(float) Constant 1065353216
+              11:    7(fvec4) ConstantComposite 10 10 10 10
+         4(main):           2 Function None 3
+               5:             Label
+                              Store 9(gl_FragColor) 11
+                              Return
+                              FunctionEnd