nv50: use BREAKADDR and BREAK for loops

nv50: use BREAKADDR and BREAK for loops

This is easier, we don't have to record all the BRK branches
to fill in the address at ENDLOOP - which wasn't done before
and thus made all but the last BRK branch jump to the start.

Also, we don't risk reactivting threads that already left
the loop if we join threads after a conditional with a BRK
(although it might be wiser to remove the join altogether
in such cases).