If I hack up GCC's old jump threader to avoid threading across backedges 
and instead let the FSM threader handle that case, then we end up with 
cases where the FSM threader creates irreducible loops with marginal 
benefit.

This can be seen in ssa-dom-thread-2{d,e,f}.c.

We've long avoided such threads in the old jump threader.  We generally 
want to avoid them in the FSM threader as well.  The only case where 
we're going to allow them is when we're able to eliminate a multi-way 
branch from the loop.

Bootstrapped and regression tested on x86_64-linux-gnu.  Also tested the 
above mentioned testcases with my hacked up compiler.

Installed on the trunk.

Jeff