Update isl to isl-0.17.1-57-g1879898

With this update the isl AST generation extracts disjunctive constraints early
on. As a result, code that previously resulted in two branches with (close-to)
identical code within them:

  if (P <= -1) {
    for (int c0 = 0; c0 < N; c0 += 1)
      Stmt_store(c0);
  } else if (P >= 1)
    for (int c0 = 0; c0 < N; c0 += 1)
       Stmt_store(c0);

results now in only a single branch body:

  if (P <= -1 || P >= 1)
    for (int c0 = 0; c0 < N; c0 += 1)
       Stmt_store(c0);

This resolves http://llvm.org/PR27559

Besides the above change, this isl update brings better simplification of
sets/maps containing existentially quantified dimensions and fixes a bug in
isl's coalescing.

llvm-svn: 272500