[MLIR][Presburger] add iterVarKind for convenient iterating over variables (#152091)

I find myself doing this alot

```
for (unsigned varIndex = rel.getVarKindOffset(VarKind::Domain);
     varIndex < rel.getVarKindEnd(VarKind::Domain); ++varIndex) {
  ...
}
```

Adding this convenience method so I can instead do

```
for (unsigned varIndex : rel.iterVarKind(VarKind::Domain)) {
  ...
}
```

---------

Co-authored-by: Jeremy Kun <j2kun@users.noreply.github.com>

mlir/include/mlir/Analysis/Presburger/IntegerRelation.h

@@ -20,6 +20,7 @@
 #include "mlir/Analysis/Presburger/PresburgerSpace.h"
 #include "mlir/Analysis/Presburger/Utils.h"
 #include "llvm/ADT/DynamicAPInt.h"
+#include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/LogicalResult.h"
 #include <optional>
@@ -268,6 +269,13 @@ public:
     return space.getVarKindEnd(kind);
   }
 
+  /// Return an interator over the variables of the specified kind
+  /// starting at the relevant offset. The return type is auto in
+  /// keeping with the convention for iterators.
+  auto iterVarKind(VarKind kind) {
+    return llvm::seq(getVarKindOffset(kind), getVarKindEnd(kind));
+  }
+
   /// Get the number of elements of the specified kind in the range
   /// [varStart, varLimit).
   unsigned getVarKindOverlap(VarKind kind, unsigned varStart,

mlir/unittests/Analysis/Presburger/IntegerRelationTest.cpp

@@ -16,6 +16,7 @@
 
 using namespace mlir;
 using namespace presburger;
+using ::testing::ElementsAre;
 
 TEST(IntegerRelationTest, getDomainAndRangeSet) {
   IntegerRelation rel = parseRelationFromSet(
@@ -702,3 +703,14 @@ TEST(IntegerRelationTest, rangeProductSymbols) {
 
   EXPECT_TRUE(expected.isEqual(rangeProd));
 }
+
+TEST(IntegerRelationTest, getVarKindRange) {
+  IntegerRelation r1 = parseRelationFromSet(
+      "(i1, i2, i3, i4, i5) : (i1 >= 0, i2 >= 0, i3 >= 0, i4 >= 0, i5 >= 0)",
+      2);
+  SmallVector<unsigned> actual;
+  for (unsigned var : r1.iterVarKind(VarKind::Range)) {
+    actual.push_back(var);
+  }
+  EXPECT_THAT(actual, ElementsAre(2, 3, 4));
+}