[compiler] Replace String.fromCharCode().length by 1

Change-Id: I35d52bb53379023123a1c5cd25f8e7f6fe21a3f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3906215Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83358}

src/compiler/typed-optimization.cc

@@ -580,6 +580,11 @@ Reduction TypedOptimization::ReduceStringLength(Node* node) {
       // The first value input to the {input} is the resulting length.
       return Replace(input->InputAt(0));
     }
+    case IrOpcode::kStringFromSingleCharCode: {
+      // Note that this isn't valid for StringFromCodePointAt, since it the
+      // string it returns can be 1 or 2 characters long.
+      return Replace(jsgraph()->Constant(1));
+    }
     default:
       break;
   }