HLSL: Fix #803: Add shape conversions to the constant-initializer path.

Test/hlsl.shapeConv.frag

@@ -28,5 +28,7 @@ float4 PixelShaderFunction(float4 input, float f) : COLOR0
     f1.x;
     f1.xxx;
 
-    return input;
+    const float4 f4 = 3.0;
+
+    return input * f4;
 }