The fold_using_range operand fetching mechanism has a variety of modes.  
The "normal" mechanism simply invokes the current or supplied 
range_query to satisfy fetching current range info for any ssa-names 
used during the evalaution of the statement,

I also added support for fur_list which allows a list of ranges to be 
supplied which is used to satisfy ssa-names as they appear in the stmt.  
Once the list is exhausted, then it reverts to using the range query.

This allows us to fold a stmt using whatever values we want. ie,

a_2 = b_3 + c_4


i can call fold_stmt (r, stmt, [1,2],  [4,5])

and a_2 would be calculated using [1,2] for the first ssa_name, and 
[4,5] for the second encountered name.  This allows us to manually fold 
stmts when we desire.

There was a bug in the implementation of fur_list where it was using the 
supplied values for *any* encountered operand, not just ssa_names.

The PHI analyzer is the first consumer of the fur_list API, and was 
tripping over this.


     <bb 3> [local count: 1052266993]:
   # a_lsm.12_29 = PHI <iftmp.1_11(4)>
   iftmp.1_15 = 3 / a_lsm.12_29;

   <bb 4> [local count: 1063004408]:
   # iftmp.1_11 = PHI <iftmp.1_15(3), 2(2)>
   # ivtmp_2 = PHI <ivtmp_14(3), 253(2)>
   ivtmp_36 = ivtmp_2 - 1;
   if (ivtmp_36 != 0)
     goto <bb 3>; [98.99%]
   else
     goto <bb 5>; [1.01%]

It detemined that the initial value of iftmp.1_11 was [2, 2] (from the 
edge 2->4), and that the only modifying statement is
iftmp.1_15 = 3 / a_lsm.12_29;

One of the things it tries to do is determine is if a few iterations 
feeding the initial value and combining it with the result of the 
statement converge, thus providing a complete initial range.  Its uses 
fold_range supplying the value for the ssa-operand directly..  but 
tripped over the bug.

So for the first iteration, instead of calculating   _15 = 3 / [2,2]  
and coming up with [1,1],   it was instead calculating [2,2]/VARYING, 
and coming up with [-2, 2].  Next pass of the iteration checker then 
erroneously calculated [-2,2]/VARYING and the result was [-2,2] and 
convergence was achieved, and the initial value of the PHI set to[-2, 2] 
... incorrectly.  and of course bad things happened.

This patch fixes fur_list::get_operand to check for an ssa-name before 
it pulling a value from the supplied list.  With this, no partlculary 
good starting value for the PHI node can be determined.

Andrew