Before we would do type analysis starting from the function entry
(using the specialization of the previous function).  This makes things
pretty complicated because we can infer different types than we are OSRing
with!  Ex if the type analysis determines that we should speculate in an
earlier BB, the types we have now might not reflect that speculation.

So instead, start the type analysis starting from the BB that the OSR starts at.
Should also have the side-benefit of requiring less type analysis work.

But this should let us get rid of the OSR-entry guarding, and the rest of
the old deopt system!