Add missing codegen for minusequals and etc. Do proper type checking on else blocks. Spread single arg constructors not supported in hlsl.

hlsl_builtin.ink

@@ -285,3 +285,5 @@ int4x4 :: struct {
 #foreign lerp :: (float2, float2, float) -> float2;
 #foreign lerp :: (float3, float3, float) -> float3;
 #foreign lerp :: (float4, float4, float) -> float4;
+
+#foreign fmod :: (float, float) -> float;