[mlir] Decrease stack allocations in Presburger's Matrix

[tpopp]

Summary

The class set a SmallVector stack allocation size to 64 elements which
is uncommonly large. These structures are then used extensively and
copied often in functions which led to stack frame sizes considered
excessively large for some use cases.

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

private:
   /// The current number of rows, columns, and reserved columns. The underlying
   /// data vector is viewed as an nRows x nReservedColumns matrix, of which the
   /// first nColumns columns are currently in use, and the remaining are
   /// reserved columns filled with zeros.
   unsigned nRows, nColumns, nReservedColumns;
 
   /// Stores the data. data.size() is equal to nRows * nReservedColumns.
   /// data.capacity() / nReservedColumns is the number of reserved rows.
-  SmallVector<int64_t, 64> data;
+  SmallVector<int64_t, 16> data;
 };
 
 } // namespace presburger
 } // namespace mlir
 
 #endif // MLIR_ANALYSIS_PRESBURGER_MATRIX_H