This class implements the processor context for Constant Propagate.
ConstantPropagateProcCtx keeps track of propagated constants in a column->const map for each
operator, enabling constants to be revolved across operators.
Get propagated constant map from parents.
Traverse all parents of current operator, if there is propagated constant (determined by
assignment expression like column=constant value), resolve the column using RowResolver and add
it to current constant map.
op - operator getting the propagated constants.
map of ColumnInfo to ExprNodeDesc. The values of that map must be either
ExprNodeConstantDesc or ExprNodeNullDesc.