Allow === and == to generate inputs at the use site when compiling to a single comparison

lib/compiler/implementation/ssa/codegen_helpers.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Instead of emitting each SSA instruction with a temporary variable
  * mark instructions that can be emitted at their use-site.
  * For example, in:
  *   t0 = 4;
  *   t1 = 3;
@@ skipping 34 matching lines @@
   void visitIs(HIs instruction) {}
 
   // A check method must not have its input generated at use site,
   // because it's using it multiple times.
   void visitCheck(HCheck instruction) {}
 
   // A type guard should not generate its input at use site, otherwise
   // they would not be alive.
   void visitTypeGuard(HTypeGuard instruction) {}
 
-  // If an equality operation is builtin it must not have its input generated at
-  // use site, because it's using it multiple times (because of null/undefined).
+  // If an equality operation is builtin it must only have its inputs generated
+  // at use site if it does not require an expression with repeated uses
+  // (because of null / undefined).
   void visitEquals(HEquals instruction) {
     if (!instruction.builtin) super.visitEquals(instruction);
-    // Otherwise do nothing.
+    if (singleIdentityComparison(instruction.left, instruction.right) != null) {
+      super.visitEquals(instruction);
+    }
+    // Do nothing.
   }
 
-  // Identity operations must not have its input generated at use site, because
-  // it's using it multiple times (because of null/undefined).
-  void visitIdentity(HIdentity instruction) {}
+  // An identity operation must only have its inputs generated at use site if
+  // does not require an expression with multiple uses (because of null /
+  // undefined).
+  void visitIdentity(HIdentity instruction) {
+    if (singleIdentityComparison(instruction.left, instruction.right) != null) {
+      super.visitIdentity(instruction);
+    }
+    // Do nothing.
+  }
 
   void visitTypeConversion(HTypeConversion instruction) {
     if (!instruction.isChecked) {
       markAsGenerateAtUseSite(instruction);
     } else if (instruction.isCheckedModeCheck) {
       // Checked mode checks compile to code that only use their input
       // once, so we can safely visit them and try to merge the input.
       visitInstruction(instruction);
     }
   }
@@ skipping 285 matching lines @@
   };
 }
 
 class JSBinaryOperatorPrecedence {
   final int left;
   final int right;
   const JSBinaryOperatorPrecedence(this.left, this.right);
   // All binary operators (excluding assignment) are left associative.
   int get precedence() => left;
 }