nv50/ir: deal with loops with no breaks

nv50/ir: deal with loops with no breaks

For example if there are only returns, the break bb will not end up part
of the CFG. However there will have been a prebreak already emitted for
it, and when hitting the RET that comes after, we will try to insert the
current (i.e. break) BB into the graph even though it will be
unreachable. This makes the SSA code sad.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>