[turbofan] Introduce dedicated StringIndexOf 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 2132d9a493a4ad463e4ee3dd5f6e2a4ffaf4b8f3..0b36f402c1d67cce290387c9aae7a1ff0fc4b986 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -2240,6 +2240,13 @@ class RepresentationSelector {
                   MachineRepresentation::kTaggedPointer);
         return;
       }
+      case IrOpcode::kStringIndexOf: {
+        ProcessInput(node, 0, UseInfo::AnyTagged());
+        ProcessInput(node, 1, UseInfo::AnyTagged());
+        ProcessInput(node, 2, UseInfo::TaggedSigned());
+        SetOutput(node, MachineRepresentation::kTaggedSigned);
+        return;
+      }
 
       case IrOpcode::kCheckBounds: {
         Type* index_type = TypeOf(node->InputAt(0));