HLSL: fix for flattening assignments from non-symbol R-values.

If a member-wise assignment from a non-flattened struct to a flattened struct sees a complex R-value
(not a symbol), it now creates a temporary to hold that value, to avoid repeating the R-value.
This avoids, e.g, duplicating a whole function call.  Also, it avoids re-using the AST node, making a
new one for each member inside the member loop.

The latter (re-use of AST node) was also an issue in the GetDimensions intrinsic decomposition,
so this PR fixes that one too.

Test/hlsl.flatten.return.frag

@@ -0,0 +1,18 @@
+
+struct PS_OUTPUT
+{
+    float4 color : SV_Target0;
+    float other_struct_member1;
+    float other_struct_member2;
+    float other_struct_member3;
+};
+
+PS_OUTPUT Func1()
+{
+    return PS_OUTPUT(float4(1), 2, 3, 4);
+}
+
+PS_OUTPUT main()
+{
+    return Func1();
+}