HLSL: Add shape conversions for scalar -> vector assigments.

Also, this allows turning on the error check for a failed assigment
when parsing.

This makes 39 HLSL tests have a working assignment that was previously
silently dropped, due to lack of this functionality.

Test/hlsl.shapeConv.frag

@@ -0,0 +1,13 @@
+float4 PixelShaderFunction(float4 input, float f) : COLOR0
+{
+    float4 v;
+    v = 1;
+    v = 2.0;
+    v = f;
+    float3 u;
+    u = float(1);
+    u = float(2.0);
+    u = float(f);
+
+    return input;
+}