[IRCE] Fix corner case with Start = INT_MAX

When iterating through loop

  for (int i = INT_MAX; i > 0; i--)

We fail to generate the pre-loop for it. It happens because we use the
overflown value in a comparison predicate when identifying whether or not
we need it.

In old logic, we used SLE predicate against Greatest value which exceeds all
seen values of the IV and might be overflown. Now we use the GreatestSeen
value of this IV with SLT predicate.

Also added a test that ensures that a pre-loop is generated for such loops.

Differential Revision: https://reviews.llvm.org/D35347

llvm-svn: 308001