Disabling codegen on parquet scanner causes some queries' performance to regress

The regression was caused by https://github.com/cloudera/Impala/commit/0686cd9c3ed7ae48d5bd4fe602266034ef871ffc, which disables codegen on any plan fragments with no codegen-enabled operators (e.g., a scan over a parquet table feeding directly into an exchange). This speeds up some queries by removing codegen overhead, but can slow down queries where the predicates on a parquet scanner benefit from codegen.

TPCDS-Q8 is definitely affected by this, and TPCDS-Q47 and TPCH-Q16 may be as well (I haven't dug into it).