nir: Properly clean up CF nodes when we remove them

nir: Properly clean up CF nodes when we remove them

Previously, if you remved a CF node that still had instructions in it, none
of the use/def information from those instructions would get cleaned up.
Also, we weren't removing if statements from the if_uses of the
corresponding register or SSA def.  This commit fixes both of these
problems

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>