[turbofan] Add support for String.prototype.charCodeAt/charAt.

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 09a157f93564319d04276a671dfa0cf7759e6434..9e1cf22543af90ff5929e016b0955e4ed84da551 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -822,10 +822,17 @@ class RepresentationSelector {
       return MachineRepresentation::kWord32;
     } else if (type->Is(Type::Boolean())) {
       return MachineRepresentation::kBit;
-    } else if (type->Is(Type::Number())) {
-      return MachineRepresentation::kFloat64;
     } else if (use.IsUsedAsFloat64()) {
       return MachineRepresentation::kFloat64;
+    } else if (type->Is(
+                   Type::Union(Type::SignedSmall(), Type::NaN(), zone()))) {
+      // TODO(turbofan): For Phis that return either NaN or some Smi, it's
+      // beneficial to not go all the way to double, unless the uses are
+      // double uses. For tagging that just means some potentially expensive
+      // allocation code; we might want to do the same for -0 as well?
+      return MachineRepresentation::kTagged;
+    } else if (type->Is(Type::Number())) {
+      return MachineRepresentation::kFloat64;
     } else if (type->Is(Type::Internal())) {
       // We mark (u)int64 as Type::Internal.
       // TODO(jarin) This is a workaround for our lack of (u)int64
@@ -1936,6 +1943,11 @@ class RepresentationSelector {
         }
         return;
       }
+      case IrOpcode::kStringCharCodeAt: {
+        VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
+                   MachineRepresentation::kWord32);
+        return;
+      }
       case IrOpcode::kStringFromCharCode: {
         VisitUnop(node, UseInfo::TruncatingWord32(),
                   MachineRepresentation::kTagged);