When a statement is first processed, any SSA_NAMEs that are dependencies 
are cached for quick future access.

if we ;later rewrite the statement (say propagate a constant into it), 
its possible the ssa-name in this cache is no longer active.   Normally 
this is not a problem, but the changed to may_recompute_p forgot to take 
that into account, and was checking a dependency from the cache that was 
in the SSA_NAME_FREE_LIST. It thus had no SSA_NAME_DEF_STMT when we were 
expecting one.

This patch simply rejects dependencies from consideration if they are in 
the free list.

Bootstrapping on x86_64-pc-linux-gnu  and presuming no regressio0ns, OK 
for trunk?

Andrew