our use of equivalences on range-on-entry calculations cause an issue 
through a PHI node when a back edge is involved.  ie
    a = VARYING
    <...>
bb5
    b = PHI <UNDEFINED(2), a (5)>
bb6
    if (a != 0)
      goto bb5

since the value of b is undefined on the edge 2->5, we ignore it. The 
range of a on the edge 6->5 is ~[0,0]
we calculate the range of b to be ~[0,0].   we also provide an 
equivalency between a and b.

Unfortunately the on-entry code looks at equivalencies, and says, "hey, 
a and b are equivalent, so we can use the range of b instead"

So it now thinks a is ~[0,0] and folds away the condition.

The problem is that b can be considered equivalent to a, but the 
converse is not true, because there is a path (2->5) upon which a is not 
equivalent to b.  we have no way to represent a one way equivalence at 
the moment. This patch avoid using that relation in range-on-entry 
calculations.

Perhaps next release I'll add a specific kind of one way equivalence for 
this kind of situation.

Bootstraps on x86_64-pc-linux-gnu with no regressions. OK?

Andrew