Add link-time cross stage optimization test

Test/link.crossStageOptimization.frag

@@ -0,0 +1,24 @@
+#version 440
+
+layout(location = 0) in vec4 a0; // accessed
+layout(location = 1) in vec4 a1; // not accessed
+layout(location = 2) in vec4 a2; // accessed
+layout(location = 3) in vec4 a3; // not accessed
+
+layout(location = 0) out vec4 oColor;
+
+void main()
+{
+    vec4 temp = vec4(1.0);
+    if (true)
+    {
+        temp *= a0;
+        temp *= a2;
+    }
+    if (false)
+    {
+        temp *= a1;
+        temp *= a3;
+    }
+    oColor = temp;
+}