The path oracles killing_def () routine was removing an ssa-name from 
each equivalence in the set.  This was busy work that did not need to be 
done.

when checking for an equivalence between A and B, the path oracle 
requires that A be in B's set and B be in A's set.  By setting the 
equivalence set for A to contain *just* A, there is no need to visit all 
the equivalences and remove them.

Aldy ran it thru the thread counter, and there was no difference in the 
number of threads found. The testcase in the PR on my machine ran in the 
following times:

there no difference i the default time, but with the specified option  
-O2 --param max-jump-thread-duplication-stmts=30 :

before patch:

backwards jump threading           :  11.64 ( 92%)   0.00 (  0%) 11.71 ( 
91%)    24  (  0%)
TOTAL                           :  12.70          0.07 12.83           47M

After patch:

backwards jump threading           :   1.96 ( 65%)   0.01 ( 10%)   1.99 
( 64%)    24  (  0%)
TOTAL                           :   3.00          0.10 3.12           47M

Clearly more work to do, but much better for a start.  next up, why is 
compute_ranges-in_block () taking over 50% of the time now.

This was bootstrapped on x86_64-pc-linux-gnu with no regressions.  pushed.

Andrew