In order to correctly compile

struct s {
  double x1;
  float x2;
};
__attribute__((regparm(3))) struct s f(int a, int b, int c);
void g(void) {
  f(41, 42, 43);
}

We need to be able to represent passing the address of s to f (sret) in a
register (inreg). Turns out that all that is needed is to not mark them as
mutually incompatible.

llvm-svn: 160695