If a relation is available, calculate overflow and normal ranges. Then 
apply as appropriate.

This patch implements operator_plus::op1/op2_range and 
operator_minus::op1_range to utilize any relation passed into properly 
reflect the range.

If the relation between the LHS and the operand being calculated is one 
of <,<=,>,>=, then determine what the overflow and normal ranges are for 
this type, and reflect those in the operand being calculated.

With this patch, we can move the testcase for PR 79095 to an evrp test 
instead of vrp1, so we resolve it much earlier.  This testcase tests 
various overflow conditions to ensure we can detect and propagate 
overflow conditions.  ie, it has a series of tests similar to:

unsigned
f1 (unsigned a, unsigned b)
{
   b = a + 1;
   if (b < a)
     {
       arf (a, b);
       return 42;
     }
   baz (a, b);
   return b;
}

It tests that 'baz' remains a call using symbolic names, and that 'arf' 
can be folded to constant arguments.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew