[clang-tidy] SimplifyBooleanExprCheck - use cast<> instead of dyn_cast<> to...

[clang-tidy] SimplifyBooleanExprCheck - use cast<> instead of dyn_cast<> to avoid dereference of nullptr

The IfStmt pointer is always referenced inside the replaceCompoundReturnWithCondition call, so assert the cast is correct instead of returning nullptr