Fix some minor issues in cfg_{optimization,to_tree}.py

kernel/modelverse_jit/cfg_optimization.py

@@ -5,7 +5,6 @@ import modelverse_jit.cfg_ir as cfg_ir
 import modelverse_jit.cfg_dominators as cfg_dominators
 import modelverse_jit.cfg_ssa_construction as cfg_ssa_construction
 import modelverse_jit.cfg_data_structures as cfg_data_structures
-import modelverse_jit.tree_ir as tree_ir
 import modelverse_kernel.primitives as primitive_functions
 
 def is_empty_block(block):

kernel/modelverse_jit/cfg_to_tree.py

@@ -1,6 +1,5 @@
 """Lowers CFG-IR to tree-IR."""
 
-from collections import defaultdict
 import modelverse_jit.cfg_ir as cfg_ir
 import modelverse_jit.tree_ir as tree_ir
 import modelverse_jit.runtime as jit_runtime