Scala Codegen generates code for MapVals when simple types are being used

[4tXJ7f]

In some cases, the backend produces code for MapVals even though it deals with simple types. Example:

[error] test/gen/tpch1.scala:78: error: value v is not a member of String
[error]       if (SUM_QTY_SUM_BASE_PRICE_SUM_DISC_PRICE_SUM_CHARGE_AVG_QTY_AVG_PRICE_AVG_DISC_COUNT_ORDER.get((lineitem_returnflag_1.v,lineitem_linestatus_1.v))==0) SUM_QTY_SUM_BASE_PRICE_SUM_DISC_PRICE_SUM_CHARGE_AVG_QTY_AVG_PRICE_AVG_DISC_COUNT_ORDER.set((lineitem_returnflag_1.v,lineitem_linestatus_1.v),v3);
[error]                                                                                                                              ^
[error] test/gen/tpch1.scala:150: error: value v is not a member of String
[error]       if (SUM_QTY_SUM_BASE_PRICE_SUM_DISC_PRICE_SUM_CHARGE_AVG_QTY_AVG_PRICE_AVG_DISC_COUNT_ORDER.get((lineitem_returnflag_1.v,lineitem_linestatus_1.v))==0) SUM_QTY_SUM_BASE_PRICE_SUM_DISC_PRICE_SUM_CHARGE_AVG_QTY_AVG_PRICE_AVG_DISC_COUNT_ORDER.set((lineitem_returnflag_1.v,lineitem_linestatus_1.v),v13);