SPV: Handle nested opaque types as function parameters.

Test/spv.functionNestedOpaque.vert

@@ -0,0 +1,26 @@
+#version 450
+
+uniform struct S {
+     sampler2D s;
+} si;
+
+void foo(sampler2D t)
+{
+    texture(t, vec2(0.5));
+}
+
+void barc(const S p)
+{
+    foo(p.s);
+}
+
+void bar(S p)
+{
+    foo(p.s);
+}
+
+void main()
+{
+    barc(si);
+    bar(si);
+}