We don't need to set up the shared space for hash join instrumentation data
if instrumentation hasn't been requested.  Let's follow the example of the
similar Sort node code and save a few cycles by skipping that when we can.

This reverts commit d59ff4ab and instead allows us to use the safer choice
of passing noError = false to shm_toc_lookup in ExecHashInitializeWorker,
since if we reach that call there should be a TOC entry to be found.

Thomas Munro

Discussion: https://postgr.es/m/E1ehkoZ-0005uW-43%40gemulon.postgresql.org