I think we changed what we protect with IsValidFrame over the course of
our refactorings. Take a look at the code before we started:
Note that the code path that can potentially result in a fast unwind is
protected by IsValidFrame (for MIPS). This makes sense, since fast
unwind requires the frame info (bp, stack_top, stack_bottom) to work.
The current code only protects the slow unwind.