[DenseMap] Don't capture the BucketEnd pointer before an operation that might...

[DenseMap] Don't capture the BucketEnd pointer before an operation that might change the number of buckets.

This code was added in 887efa51 to
fix reverse iteration.

The call to InsertIntoBucket/InsertIntoBucketWithLookup can change
the number of buckets which will invalidate the BucketEnd. So
don't cache it and calculate it when creating the iterator.