[turbofan] Add dedicated test for check constant folding.

src/compiler/js-typed-lowering.cc

Index: src/compiler/js-typed-lowering.cc
diff --git a/src/compiler/js-typed-lowering.cc b/src/compiler/js-typed-lowering.cc
index 4c3318135287e9988c2be00d4a84d048c582502c..213a8a414660f87e7f9c685689c07dd3b8c85d8c 100644
--- a/src/compiler/js-typed-lowering.cc
+++ b/src/compiler/js-typed-lowering.cc
@@ -1853,6 +1853,10 @@ Reduction JSTypedLowering::Reduce(Node* node) {
   // result value and can simply replace the node if it's eliminable.
   if (!NodeProperties::IsConstant(node) && NodeProperties::IsTyped(node) &&
       node->op()->HasProperty(Operator::kEliminatable)) {
+    // We can only constant-fold nodes here, that are known to not cause any
+    // side-effect, may it be a JavaScript observable side-effect or a possible
+    // eager deoptimization exit (i.e. {node} has an operator that doesn't have
+    // the Operator::kNoDeopt property).
     Type* upper = NodeProperties::GetType(node);
     if (upper->IsInhabited()) {
       if (upper->IsConstant()) {