Cleanup stale TODO comments

Phi instructions in SSA unit is previously allocated via builtin
malloc, and wasn't freeing after SSA unit completed. Later, we
replaced allocator with arena allocator, which guaranteed its lifetime
is within the compilation, thus the TODO comment is removed as resolved.

Close #275.

Author: ChAoSUnItY

src/ssa.c

@@ -903,8 +903,6 @@ void bb_unwind_phi(func_t *func, basic_block_t *bb)
         for (phi_operand_t *operand = insn->phi_ops; operand;
              operand = operand->next)
             append_unwound_phi_insn(operand->from, insn->rd, operand->var);
-        /* TODO: Release memory allocated for phi instruction to prevent leaks
-         */
     }
 
     bb->insn_list.head = insn;