broadcom/vc5: Properly schedule the thread-end THRSW.

broadcom/vc5: Properly schedule the thread-end THRSW.

This fills in the delay slots of thread end as much as we can (other than
being cautious about potential TLBZ writes).

In the process, I moved the thread end THRSW instruction creation to the
scheduler.  Once we start emitting THRSWs in the shader, we need to
schedule the thread-end one differently from other THRSWs, so having it in
there makes that easy.