nir/cf: Alter block successors before adding a fake link.

nir/cf: Alter block successors before adding a fake link.

Consider the case of "while (...) { break }".  Or in NIR:

        block block_0 (0x7ab640):
        ...
        /* succs: block_1 */
        loop {
                block block_1:
                /* preds: block_0 */
                break
                /* succs: block_2 */
        }
        block block_2:

Calling nir_handle_remove_jump(block_1, nir_jump_break) will remove the break.
Unfortunately, it would mangle the predecessors and successors.

Here, block_2->predecessors->entries == 1, so we would create a fake
link, setting block_1->successors[1] = block_2, and adding block_1 to
block_2's predecessor set.  This is illegal: a block cannot specify the
same successor twice.  In particular, adding the predecessor would have
no effect, as it was already present in the set.

We'd then call unlink_block_successors(), which would delete the fake
link and remove block_1 from block_2's predecessor set.  It would then
delete successors[0], and attempt to remove block_1 from block_2's
predecessor set a second time...except that it wouldn't be present,
triggering an assertion failure.

The fix appears to be simple: simply unlink the block's successors and
recreate them to point at the correct blocks first.  Then, add the fake
link.  In the above example, removing the break would cause block_1 to
have itself as a successor (as it becomes an infinite loop), so adding
the fake link won't cause a duplicate successor.

v2: Add comments (requested by Connor Abbott) and fix commit message.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>