nir/cf: fix link_blocks() when there are no successors

nir/cf: fix link_blocks() when there are no successors

When we insert a single basic block A into another basic block B, we
will split B into C and D, insert A in the middle, and then splice
together C, A, and D. When we splice together C and A, we need to move
the successors of A into C -- except A has no successors, since it
hasn't been inserted yet. So in move_successors(), we need to handle the
case where the block whose successors are to be moved doesn't have any
successors. Fixing link_blocks() here prevents a segfault and makes it
work correctly.

Signed-off-by: Connor Abbott <connor.w.abbott@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>