I think the ip we receive is the return address to that stack frame
(at least in the non-signal case).  This makes things a bit weird
since the "is this ip in this range" tests have a different half-openness
than they normally do.  At some point an "ip = ip - 1" snuck in, which
I think was to address this issue, but I think it's better to not
change the ip -- ie the resulting address is not independently useful (it's
in the middle of an instruction); we could pass it around as "ip_minus_one",
but instead just try converting the tests to be better.