[turbofan] Disable incorrect fast-path of String.p.codePointAt

For M65 I added a fast-path for String.p.charCodeAt, which behaves
incorrectly for wide characters. This patch disables the incorrect
fast-path. This is likely no performance regression against M64,
as M64 did not have a fast path for String.p.charCodeAt.

Bug: v8:7371
Change-Id: I8ea9c5da5a583138d87ccb8ffe6d61eba16630b9
Reviewed-on: https://chromium-review.googlesource.com/886782
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50873}

src/compiler/simplified-lowering.cc

@@ -2358,19 +2358,9 @@ class RepresentationSelector {
         return;
       }
       case IrOpcode::kStringCodePointAt: {
-        Type* string_type = TypeOf(node->InputAt(0));
-        if (string_type->Is(Type::SeqString())) {
-          VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
-                     MachineRepresentation::kWord32);
-          if (lower()) {
-            NodeProperties::ChangeOp(node,
-                                     simplified()->SeqStringCodePointAt());
-          }
-        } else {
         // TODO(turbofan): Allow builtins to return untagged values.
         VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
                    MachineRepresentation::kTaggedSigned);
-        }
         return;
       }
       case IrOpcode::kStringFromCharCode: {

test/mjsunit/constant-folding-2.js

@@ -259,7 +259,8 @@ test(function stringCharCodeAt() {
 }, 10);
 
 test(function stringCodePointAt() {
-  assertSame(65533, "äϠ�".codePointAt(2));
+  assertSame(65533, "äϠ�𝌆".codePointAt(2));
+  assertSame(119558, "äϠ�𝌆".codePointAt(3));
   assertSame(undefined, "äϠ�".codePointAt(-1));
   assertSame(undefined, "äϠ�".codePointAt(4));
   assertSame(992, "äϠ�".codePointAt(1.1));