nir/cf: clean up jumps when cleaning up CF nodes

nir/cf: clean up jumps when cleaning up CF nodes

We may delete a control flow node which contains structured jumps to
other parts of the program. We need to remove the jump as a predecessor,
as well as remove any phi node sources which reference it. Right now,
the same problem exists for blocks that don't end in a jump instruction,
but with the new API it shouldn't be an issue, since blocks that don't
end in a jump must either point to another block in the same extracted
CF list or not point to anything at all.

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