[MLIR][Presburger] add a non-const Matrix::getRow() returning a MutableArrayRef

[Groverkss]

LGTM.

mlir/include/mlir/Analysis/Presburger/Matrix.h

public:
   /// incurring a reallocation.
   unsigned getNumReservedRows() const;
   unsigned getNumReservedColumns() const;
 
   /// Reserve enough space to resize to the specified number of rows without
   /// reallocations.
   void reserveRows(unsigned rows);
 
-  /// Get an ArrayRef corresponding to the specified row.
+  /// Get a [Mutable]ArrayRef corresponding to the specified row.
+  MutableArrayRef<int64_t> getRow(unsigned row);
   ArrayRef<int64_t> getRow(unsigned row) const;
 
   /// Insert columns having positions pos, pos + 1, ... pos + count - 1.
   /// Columns that were at positions 0 to pos - 1 will stay where they are;
   /// columns that were at positions pos to nColumns - 1 will be pushed to the
   /// right. pos should be at most nColumns.
   void insertColumns(unsigned pos, unsigned count);
   void insertColumn(unsigned pos);

mlir/lib/Analysis/Presburger/Matrix.cpp

void Matrix::swapColumns(unsigned column, unsigned otherColumn) {
   assert((column < getNumColumns() && otherColumn < getNumColumns()) &&
          "Given column out of bounds");
   if (column == otherColumn)
     return;
   for (unsigned row = 0; row < nRows; row++)
     std::swap(at(row, column), at(row, otherColumn));
 }
 
+MutableArrayRef<int64_t> Matrix::getRow(unsigned row) {
+  return {&data[row * nReservedColumns], nColumns};
+}
+
 ArrayRef<int64_t> Matrix::getRow(unsigned row) const {
   return {&data[row * nReservedColumns], nColumns};
 }
 
 void Matrix::insertColumn(unsigned pos) { insertColumns(pos, 1); }
 void Matrix::insertColumns(unsigned pos, unsigned count) {
   if (count == 0)
     return;