SPV: When passing structs of opaque types, flatten and pass the members instead.

This avoids either A) needing uniformConstant struct, or
B) initializing a struct with opaque members, as writing them is not
allowed.

Test/hlsl.flattenOpaque.frag

@@ -0,0 +1,40 @@
+struct os {
+    sampler2D s2D;
+};
+
+struct os2 {
+    sampler2D s2D;
+    Texture2D tex;
+};
+
+Texture2D tex;
+os s;
+os2 s2;
+
+float4 osCall1(os s)
+{
+    return tex.Sample(s.s2D, float2(0.2, 0.3));
+}
+
+float4 osCall2(os s, float2 f2)
+{
+    return tex.Sample(s.s2D, f2);
+}
+
+float4 os2Call1(os2 s)
+{
+    return s.tex.Sample(s.s2D, float2(0.2, 0.3));
+}
+
+float4 os2Call2(os2 s, float2 f2)
+{
+    return s.tex.Sample(s.s2D, f2);
+}
+
+float4 main() : SV_TARGET0
+{
+    return osCall1(s) +
+           osCall2(s, float2(0.2, 0.3)) +
+           os2Call1(s2) +
+           os2Call2(s2, float2(0.2, 0.3));
+}