libstdc++: small dynamic_cast optimization

libstdc++: small dynamic_cast optimization

This change speeds up the simple benchmark below by about 40%.

struct A { virtual ~A() {} };
struct B: A { } b;
A* ap = &b;
void *sink;
int main()
{
  for (long i = 0; i < 4000000000L; ++i)
    sink = dynamic_cast<B*>(ap);
}

libstdc++-v3/ChangeLog:

	* libsupc++/dyncast.cc (__dynamic_cast): Avoid virtual function
	call in simple success case.