nir/cf: handle jumps in split_block_end()

nir/cf: handle jumps in split_block_end()

Before, we would only split a block with a jump at the end if we were
inserting something after a block with a jump, which never happened in
practice. But now, we want to use this to extract control flow lists
which may end in a jump, in which case we really need to do the correct
patching up. As a side effect, when removing jumps we now correctly
insert undef phi sources in some corner cases, which can't hurt.

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