[bug] Add default value to print_preprocessed_ir (#3292)

Author: lin-hitonami

taichi/program/compile_config.cpp

@@ -10,6 +10,7 @@ CompileConfig::CompileConfig() {
   external_optimization_level = 3;
   packed = false;
   print_ir = false;
+  print_preprocessed_ir = false;
   print_accessor_ir = false;
   print_evaluator_ir = false;
   print_benchmark_stat = false;