[turbofan] Mark side-effect-free calls to string ops as kEliminatable.

src/compiler/effect-control-linearizer.cc

Index: src/compiler/effect-control-linearizer.cc
diff --git a/src/compiler/effect-control-linearizer.cc b/src/compiler/effect-control-linearizer.cc
index 98d6832afd51b83e387e80b7a22203a32b3c19b5..d6ba32bc969cc430cb0ea6a4d416f600979247aa 100644
--- a/src/compiler/effect-control-linearizer.cc
+++ b/src/compiler/effect-control-linearizer.cc
@@ -1290,6 +1290,7 @@ EffectControlLinearizer::LowerCheckIf(Node* node, Node* frame_state,
                                       Node* effect, Node* control) {
   NodeProperties::ReplaceEffectInput(node, effect);
   NodeProperties::ReplaceControlInput(node, control);
+  DCHECK_NOT_NULL(frame_state);
   node->InsertInput(graph()->zone(), 1, frame_state);
   NodeProperties::ChangeOp(node, common()->DeoptimizeIf());
   return ValueEffectControl(node, node, node);