Re-enable optimization to avoid loading data from system memory on an
L2 cache miss if the transaction is a block vector write to the entire
cache line.
The previous version of this optimization would corrupt memory because
it was performed in the directory stage and didn't consider whether
other misses were pending for the same cache line.  The new version
will restart the request (after it passes the pending miss unit, which
will reconcile duplicated misses), and then short circuits a load
in the same way a duplicated request in the load queue does.