Seen while working on signal handling:

I see these in the various Posix signal handlers in os_xxx_yyy:

```
    // jni_fast_Get<Primitive>Field can trap at certain pc's if a GC kicks in
    // and the heap gets shrunk before the field access.
    if ((sig == SIGSEGV) || (sig == SIGBUS)) {
      address addr_slow = JNI_FastGetField::find_slowcase_pc(pc);
      if (addr_slow != (address)-1) {
        stub = addr_slow;
      }
    }
```

I think this gets executed unnecessarily often. If I understand the comment correctly, it should only be executed if
- A return stub has not yet been determined at this point
- The faulting address is in heap range