cparser fails to do function call with __builtin_isnan as argument

[GabrielRavier]

void f(int);

void g(float x)
{
  f(__builtin_isnan(x));
}

cparser outputs this :

ir/be/betranshlp.c:216: libFirm panic in be_transform_node: no transformer for Mux Is[71:12]
Aborted (core dumped)