[turbofan] Add new StringFromCharCode simplified operator.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index add24d66b97b074dfc944f7b1c642308bcebeeda..394750c88c2803408081064712cae9dd204ab712 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -1170,6 +1170,11 @@ class RepresentationSelector {
         }
         break;
       }
+      case IrOpcode::kStringFromCharCode: {
+        VisitUnop(node, UseInfo::TruncatingWord32(),
+                  MachineRepresentation::kTagged);
+        break;
+      }
       case IrOpcode::kStringToNumber: {
         VisitUnop(node, UseInfo::AnyTagged(), MachineRepresentation::kTagged);
         if (lower()) {