nir: Validate that a block doesn't have two identical successors.

nir: Validate that a block doesn't have two identical successors.

This is invalid, and causes disasters if we try to unlink successors:
removing the first will work, but removing the second copy will fail
because the block isn't in the successor's predecessor set any longer.

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