Unchecked dynamic_cast

If the dynamic_cast fails, a null pointer dereference will occur; if the dynamic_cast cannot fail, using static_cast instead offers better performance. Dynamic_cast may fail and return null if the type cast is incompatible
/src/mongo/db/pipeline/expression.cpp:6240: FORWARD_NULL 114690 Dynamic cast to pointer "dynamic_cast <mongo::ExpressionConstant *>(this->_regex->get())" can return "NULL".