[MLIR][Presburger] Matrix::insertColumns: add doc, fix lint issue, and early exit when possible

[Groverkss]

LGTM.

mlir/lib/Analysis/Presburger/Matrix.cpp

void Matrix::insertColumns(unsigned pos, unsigned count) {
   }
   nColumns += count;
 
   for (int ri = nRows - 1; ri >= 0; --ri) {
     for (int ci = nReservedColumns - 1; ci >= 0; --ci) {
       unsigned r = ri;
       unsigned c = ci;
       int64_t &dest = data[r * nReservedColumns + c];
-      if (c >= nColumns)
+      if (c >= nColumns) { // NOLINT
+        // Out of bounds columns are zero-initialized. NOLINT because clang-tidy
+        // complains about this branch being the same as the c >= pos one.
+        //
+        // TODO: this case can be skipped if the number of reserved columns
+        // didn't change.
         dest = 0;
-      else if (c >= pos + count)
+      } else if (c >= pos + count) {
+        // Shift the data occuring after the inserted columns.
         dest = data[r * oldNReservedColumns + c - count];
-      else if (c >= pos)
+      } else if (c >= pos) {
+        // The inserted columns are also zero-initialized.
         dest = 0;
-      else
+      } else {
+        // The columns before the inserted columns stay at the same (row, col)
+        // but this corresponds to a different location in the linearized array
+        // if the number of reserved columns changed.
+        if (nReservedColumns == oldNReservedColumns)
+          break;
         dest = data[r * oldNReservedColumns + c];
       }
     }
   }
+}
 
 void Matrix::removeColumn(unsigned pos) { removeColumns(pos, 1); }
 void Matrix::removeColumns(unsigned pos, unsigned count) {
   if (count == 0)
     return;
   assert(pos + count - 1 < nColumns);
   for (unsigned r = 0; r < nRows; ++r) {
     for (unsigned c = pos; c < nColumns - count; ++c)