HLSL: Fix #805: Support cast of scalars to structures.

Somewhat complex due to recognizing a general scalar, but not
replicating it for each member to avoid side effects.

Test/hlsl.scalarCast.vert

@@ -0,0 +1,37 @@
+struct VertexOut {
+    float4 position : SV_Position;
+    float2 texCoord : TEXCOORD;
+};
+VertexOut r0() {
+    const float f = 2.0;
+    return (VertexOut)f;
+}
+VertexOut r1() {
+    const float f = 2.0;
+    return (VertexOut)(f + 1.0);
+}
+VertexOut r2() {
+    const float f = 2.0;
+    return (VertexOut)(sin(f));
+}
+VertexOut r3() {
+    float f = 2.0;
+    return (VertexOut)f;
+}
+VertexOut r4() {
+    float f = 2.0;
+    return (VertexOut)(f + 1.0);
+}
+VertexOut r5() {
+    float f = 2.0;
+    return (VertexOut)(sin(f));
+}
+VertexOut main() {
+    VertexOut v0 = r0();
+    VertexOut v1 = r1();
+    VertexOut v2 = r2();
+    VertexOut v3 = r3();
+    VertexOut v4 = r4();
+    VertexOut v5 = r5();
+    return (VertexOut)1;
+}