Unortunately there is no equivalent. Postgres does not have "anonymous" blocks like SQL Server (or Oracle).
If you just want to make sure that the table is created (and don't care if it contains data), you could first do a DROP TABLE <tablename> IF EXISTS;
and then run the CREATE TABLE. Not a suitable solution if must keep the data in the table though.
You will probably need to create a function to do that (or use a <insert your favorite scripting language> script)
Check out the mailing list, there have been some suggestions on how to simulate this with PG. The searchable archive is here