Replace TypeMask.isEmpty by isEmptyOrNull, use isEmpty when it's really empty.

pkg/compiler/lib/src/ssa/nodes.dart

Index: pkg/compiler/lib/src/ssa/nodes.dart
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index c279da9d03c31918b21a85858a9cf8a8187c52e9..28647bd0c9420e693b96b6f2bca39b58495a761d 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -881,10 +881,9 @@ abstract class HInstruction implements Spannable {
 
   bool canBeNull() => instructionType.isNullable;
 
-  bool isNull() => instructionType.isEmpty && instructionType.isNullable;
-  bool isConflicting() {
-    return instructionType.isEmpty && !instructionType.isNullable;
-  }
+  bool isNull() => instructionType.isNull;
+
+  bool isConflicting() => instructionType.isEmpty;
 
   /// Returns `true` if [typeMask] contains [cls].
   static bool containsType(