sql/parser: Simplify FuncExpr handling in isConstVisitor
sql/parser: Simplify FuncExpr handling in isConstVisitor

FuncExpr's should never need to re-lookup their corresponding function
after type checking has succeeded.

glycerine/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,arjun4084346/cockroach,petermattis/cockroach,glycerine/cockroach,maxlang/cockroach,petermattis/cockroach,sunya123/cockroach,glycerine/cockroach,arjun4084346/cockroach,arjun4084346/cockroach,paperstreet/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,arjun4084346/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,maxlang/cockroach,petermattis/cockroach,sunya123/cockroach,maxlang/cockroach,arjun4084346/cockroach,sunya123/cockroach,arjun4084346/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,sunya123/cockroach,paperstreet/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,glycerine/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,petermattis/cockroach,glycerine/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,veteranlu/cockroach,maxlang/cockroach,vivekmenezes/cockroach,maxlang/cockroach,petermattis/cockroach,paperstreet/cockroach,maxlang/cockroach,sunya123/cockroach,veteranlu/cockroach,sunya123/cockroach,vaibhavarora/cockroach,petermattis/cockroach,maxlang/cockroach,petermattis/cockroach,cockroachdb/cockroach,paperstreet/cockroach,arjun4084346/cockroach,veteranlu/cockroach,arjun4084346/cockroach,paperstreet/cockroach,glycerine/cockroach,sunya123/cockroach,veteranlu/cockroach,sunya123/cockroach,vivekmenezes/cockroach