compiler: do not emit code for unnamed unused variables

If variable is unnamed and does not contain function call then it's
treated as unused and code generation may be omitted for it
initialization/declaration.

pkg/compiler/init_test.go

@@ -100,7 +100,7 @@ func TestInitWithNoGlobals(t *testing.T) {
 	func Main() int {
 		return 42
 	}`
-	v, s := vmAndCompileInterop(t, src)
+	v, s, _ := vmAndCompileInterop(t, src)
 	require.NoError(t, v.Run())
 	assertResult(t, v, big.NewInt(42))
 	require.True(t, len(s.events) == 1)