[SPARK-4829] [SQL] add rule to fold count(expr) if expr is not null

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #3676 from adrian-wang/countexpr and squashes the following commits:

dc5765b [Daoyuan Wang] add rule to fold count(expr) if expr is not null

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -210,6 +210,7 @@ object NullPropagation extends Rule[LogicalPlan] {
       case e @ GetField(Literal(null, _), _) => Literal(null, e.dataType)
       case e @ EqualNullSafe(Literal(null, _), r) => IsNull(r)
       case e @ EqualNullSafe(l, Literal(null, _)) => IsNull(l)
+      case e @ Count(expr) if !expr.nullable => Count(Literal(1))
 
       // For Coalesce, remove null literals.
       case e @ Coalesce(children) =>