[SPARK-22815][SQL] Keep PromotePrecision in Optimized Plans

## What changes were proposed in this pull request?
We could get incorrect results by running DecimalPrecision twice. This PR resolves the original found in #15048 and #14797. After this PR, it becomes easier to change it back using `children` instead of using `innerChildren`.

## How was this patch tested?
The existing test.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #20000 from gatorsmile/keepPromotePrecision.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/StreamingJoinHelper.scala

@@ -20,7 +20,7 @@ package org.apache.spark.sql.catalyst.analysis
 import scala.util.control.NonFatal
 
 import org.apache.spark.internal.Logging
-import org.apache.spark.sql.catalyst.expressions.{Add, AttributeReference, AttributeSet, Cast, CheckOverflow, Expression, ExpressionSet, GreaterThan, GreaterThanOrEqual, LessThan, LessThanOrEqual, Literal, Multiply, PreciseTimestampConversion, PredicateHelper, Subtract, TimeAdd, TimeSub, UnaryMinus}
+import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.planning.ExtractEquiJoinKeys
 import org.apache.spark.sql.catalyst.plans.logical.{EventTimeWatermark, LogicalPlan}
 import org.apache.spark.sql.catalyst.plans.logical.EventTimeWatermark._
@@ -238,6 +238,8 @@ object StreamingJoinHelper extends PredicateHelper with Logging {
           collect(child, !negate)
         case CheckOverflow(child, _) =>
           collect(child, negate)
+        case PromotePrecision(child) =>
+          collect(child, negate)
         case Cast(child, dataType, _) =>
           dataType match {
             case _: NumericType | _: TimestampType => collect(child, negate)

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/decimalExpressions.scala

@@ -70,10 +70,12 @@ case class MakeDecimal(child: Expression, precision: Int, scale: Int) extends Un
 case class PromotePrecision(child: Expression) extends UnaryExpression {
   override def dataType: DataType = child.dataType
   override def eval(input: InternalRow): Any = child.eval(input)
+  /** Just a simple pass-through for code generation. */
   override def genCode(ctx: CodegenContext): ExprCode = child.genCode(ctx)
   override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode = ev.copy("")
   override def prettyName: String = "promote_precision"
   override def sql: String = child.sql
+  override lazy val canonicalized: Expression = child.canonicalized
 }
 
 /**

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

@@ -614,7 +614,6 @@ object SimplifyCasts extends Rule[LogicalPlan] {
 object RemoveDispensableExpressions extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transformAllExpressions {
     case UnaryPositive(child) => child
-    case PromotePrecision(child) => child
   }
 }