[MLIR][Presburger] fix double increment in coalesce

[webmiche]

Summary

In the main-loop of the current coalesce implementation i was incremented
twice for some cases. This patch fixes this bug and adds a regression
testcase.

mlir/lib/Analysis/Presburger/PresburgerSet.cpp

for (unsigned i = 0; i < polyhedrons.size();) {
     ++i;
     simplices.push_back(simp);
   }
 
   // For all tuples of IntegerPolyhedrons, check whether they can be coalesced.
   // When coalescing is successful, the contained IntegerPolyhedron is swapped
   // with the last element of `polyhedrons` and subsequently erased and
   // similarly for simplices.
-  for (unsigned i = 0; i < polyhedrons.size(); ++i) {
+  for (unsigned i = 0; i < polyhedrons.size();) {
 
     // TODO: This does some comparisons two times (index 0 with 1 and index 1
     // with 0).
     bool broken = false;
     for (unsigned j = 0, e = polyhedrons.size(); j < e; ++j) {
       if (i == j)
         continue;
       if (coalescePair(i, j, polyhedrons, simplices).succeeded()) {

mlir/unittests/Analysis/Presburger/PresburgerSetTest.cpp

PresburgerSet set =
       parsePresburgerSetFromPolyStrings(1, {
                                                "(x) : (x >= 0, -x + 1 >= 0)",
                                                "(x) : (x >= 0, -x + 2 >= 0)",
                                                "(x) : (x >= 0, -x + 3 >= 0)",
                                            });
   expectCoalesce(1, set);
 }
 
+TEST(SetTest, coalesceDoubleIncrement) {
+  PresburgerSet set = parsePresburgerSetFromPolyStrings(
+      1, {
+             "(x) : (x == 0)",
+             "(x) : (x - 2 == 0)",
+             "(x) : (x + 2 == 0)",
+             "(x) : (x - 2 >= 0, -x + 3 >= 0)",
+         });
+  expectCoalesce(3, set);
+}
+
 static void
 expectComputedVolumeIsValidOverapprox(const PresburgerSet &set,
                                       Optional<uint64_t> trueVolume,
                                       Optional<uint64_t> resultBound) {
   expectComputedVolumeIsValidOverapprox(set.computeVolume(), trueVolume,
                                         resultBound);
 }