[turbofan] Eliminate unused effectful nodes during representation selection.

src/compiler/representation-change.h

Index: src/compiler/representation-change.h
diff --git a/src/compiler/representation-change.h b/src/compiler/representation-change.h
index b6cf7fa5995425d755ff1031c3f555782290069e..ab73a411db8a469e7a861fd1f29b286b4576fbb8 100644
--- a/src/compiler/representation-change.h
+++ b/src/compiler/representation-change.h
@@ -28,6 +28,7 @@ class Truncation final {
   }
 
   // Queries.
+  bool IsUnused() const { return kind_ == TruncationKind::kNone; }
   bool TruncatesToWord32() const {
     return LessGeneral(kind_, TruncationKind::kWord32);
   }