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.

gtests/Hlsl.FromFile.cpp

@@ -99,6 +99,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.entry-out.frag", "PixelShaderFunction"},
         {"hlsl.float1.frag", "PixelShaderFunction"},
         {"hlsl.float4.frag", "PixelShaderFunction"},
+        {"hlsl.flatten.return.frag", "main"},
         {"hlsl.forLoop.frag", "PixelShaderFunction"},
         {"hlsl.gather.array.dx10.frag", "main"},
         {"hlsl.gather.basic.dx10.frag", "main"},