nir/cf: split up and improve nir_handle_remove_jumps()

nir/cf: split up and improve nir_handle_remove_jumps()

Before, the process of removing a jump and wiring up the remaining block
correctly was atomic, but with the new control flow modification it's
split into two parts: first, we extract the jump, which creates a new
block with re-wired successors as well as a free-floating jump, and then
we delete the control flow containing the jump, which removes the entry
in the predecessors and any phi node sources. Split up
nir_handle_remove_jumps() to accomodate this, and add the missing
support for removing phi node sources.

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