This kinda-sorta corresponds to Bernd's 007-dw2cfi patch. Certainly
the same concepts of splitting the instruction stream into extended
basic blocks is the same. This patch does a bit better job with the
documentation. Also, I'm a bit more explicit about matching things
up with the similar code from the regular CFG routines.
What's missing at this point is any attempt to use DW_CFA_remember_state.
I've deferred that for the moment because it's easy to test the state
change code across epilogues, whereas the shrink-wrapping code is not
in this tree and section switching is difficult to force.