Improve comment for eliminating 1 metadata query / table.

Improve SQL Compiler speed by eliminating 1 metadata query / table.

For each table, view, etc. referenced by a user query, the SQL
Compiler issues several metadata queries to obtain info about
the object.  This change eliminates the first of those queries in
the normal case where the object exists.  If the object does not
exist, one additional metadata query may be done, but performance
is not that important for the error path.

Change-Id: Ic1c44fd183ff92143fabd97d61c405c21d04216b