Avoid creating result temp for is-expressions

src/Compilers/CSharp/Portable/BoundTree/BoundNode.cs

@@ -544,6 +544,14 @@ private void CheckDeclared(LocalSymbol local)
                 return null;
             }
 
+            public override BoundNode? VisitLoweredIsPatternExpression(BoundLoweredIsPatternExpression node)
+            {
+                AddAll(node.Locals);
+                base.VisitLoweredIsPatternExpression(node);
+                RemoveAll(node.Locals);
+                return null;
+            }
+
             public override BoundNode? VisitSwitchStatement(BoundSwitchStatement node)
             {
                 AddAll(node.InnerLocals);

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -2265,6 +2265,7 @@
          'is not Type t') will need to compensate for negated patterns. IsNegated is set if Pattern is the negated
          form of the inner pattern represented by DecisionDag. -->
   <Node Name="BoundIsPatternExpression" Base="BoundExpression">
+    <Field Name="Type" Type="TypeSymbol" Override="true" Null="disallow"/>
     <Field Name="Expression" Type="BoundExpression" Null="disallow"/>
     <Field Name="Pattern" Type="BoundPattern" Null="disallow"/>
     <Field Name="IsNegated" Type="bool"/>
@@ -2273,6 +2274,14 @@
     <Field Name="WhenFalseLabel" Type="LabelSymbol" Null="disallow"/>
   </Node>
 
+  <Node Name="BoundLoweredIsPatternExpression" Base="BoundExpression">
+    <Field Name="Type" Type="TypeSymbol" Override="true" Null="disallow"/>
+    <Field Name="Locals" Type="ImmutableArray&lt;LocalSymbol&gt;"/>
+    <Field Name="Statements" Type="ImmutableArray&lt;BoundStatement&gt;"/>
+    <Field Name="WhenTrueLabel" Type="LabelSymbol"/>
+    <Field Name="WhenFalseLabel" Type="LabelSymbol"/>
+  </Node>
+
   <AbstractNode Name="BoundPattern" Base="BoundNode">
     <Field Name="InputType" Type="TypeSymbol" Null="disallow"/>
     <Field Name="NarrowedType" Type="TypeSymbol" Null="disallow"/>

src/Compilers/CSharp/Portable/CodeGen/EmitExpression.cs

@@ -315,6 +315,10 @@ private void EmitExpressionCore(BoundExpression expression, bool used)
                     EmitRefValueOperator((BoundRefValueOperator)expression, used);
                     break;
 
+                case BoundKind.LoweredIsPatternExpression:
+                    EmitLoweredIsPatternExpression((BoundLoweredIsPatternExpression)expression, used);
+                    break;
+
                 case BoundKind.LoweredConditionalAccess:
                     EmitLoweredConditionalAccessExpression((BoundLoweredConditionalAccess)expression, used);
                     break;
@@ -352,6 +356,33 @@ private void EmitExpressionCore(BoundExpression expression, bool used)
             }
         }
 
+        private void EmitLoweredIsPatternExpression(BoundLoweredIsPatternExpression node, bool used)
+        {
+            _builder.AssertStackEmpty();
+
+            DefineLocals(node.Syntax, node.Locals);
+            EmitStatements(node.Statements);
+
+            if (!used)
+            {
+                _builder.MarkLabel(node.WhenTrueLabel);
+                _builder.MarkLabel(node.WhenFalseLabel);
+            }
+            else
+            {
+                var doneLabel = new object();
+                _builder.MarkLabel(node.WhenTrueLabel);
+                _builder.EmitBoolConstant(true);
+                _builder.EmitBranch(ILOpCode.Br, doneLabel);
+                _builder.AdjustStack(-1);
+                _builder.MarkLabel(node.WhenFalseLabel);
+                _builder.EmitBoolConstant(false);
+                _builder.MarkLabel(doneLabel);
+            }
+
+            FreeLocals(node.Locals);
+        }
+
         private void EmitThrowExpression(BoundThrowExpression node, bool used)
         {
             this.EmitThrow(node.Expression);
@@ -852,29 +883,39 @@ private void EmitSequenceExpression(BoundSequence sequence, bool used)
 
         private void DefineLocals(BoundSequence sequence)
         {
-            if (sequence.Locals.IsEmpty)
+            DefineLocals(sequence.Syntax, sequence.Locals);
+        }
+
+        private void DefineLocals(SyntaxNode syntax, ImmutableArray<LocalSymbol> locals)
+        {
+            if (locals.IsEmpty)
             {
                 return;
             }
 
             _builder.OpenLocalScope();
 
-            foreach (var local in sequence.Locals)
+            foreach (var local in locals)
             {
-                DefineLocal(local, sequence.Syntax);
+                DefineLocal(local, syntax);
             }
         }
 
         private void FreeLocals(BoundSequence sequence)
         {
-            if (sequence.Locals.IsEmpty)
+            FreeLocals(sequence.Locals);
+        }
+
+        private void FreeLocals(ImmutableArray<LocalSymbol> locals)
+        {
+            if (locals.IsEmpty)
             {
                 return;
             }
 
             _builder.CloseLocalScope();
 
-            foreach (var local in sequence.Locals)
+            foreach (var local in locals)
             {
                 FreeLocal(local);
             }

src/Compilers/CSharp/Portable/CodeGen/Optimizer.cs

@@ -1447,6 +1447,16 @@ public override BoundNode VisitSwitchDispatch(BoundSwitchDispatch node)
             return node.Update(boundExpression, node.Cases, node.DefaultLabel, node.LengthBasedStringSwitchDataOpt);
         }
 
+        public override BoundNode VisitLoweredIsPatternExpression(BoundLoweredIsPatternExpression node)
+        {
+            EnsureOnlyEvalStack();
+            DeclareLocals(node.Locals, stack: 0);
+            var result = base.VisitLoweredIsPatternExpression(node);
+            RecordBranch(node.WhenTrueLabel);
+            RecordBranch(node.WhenFalseLabel);
+            return result;
+        }
+
         public override BoundNode VisitConditionalOperator(BoundConditionalOperator node)
         {
             var origStack = StackDepth();

src/Compilers/CSharp/Portable/FlowAnalysis/AbstractFlowPass.cs

@@ -2973,6 +2973,11 @@ public override BoundNode VisitConditionalReceiver(BoundConditionalReceiver node
             return null;
         }
 
+        public override BoundNode VisitLoweredIsPatternExpression(BoundLoweredIsPatternExpression node)
+        {
+            return null;
+        }
+
         public override BoundNode VisitComplexConditionalReceiver(BoundComplexConditionalReceiver node)
         {
             var savedState = this.State.Clone();