[turbofan] Remove ExternalPointer hack from SimplifiedLowering.

This is no longer necessary as the JSFunction::code field is now a
proper tagged field, that points to a Code object, instead of a
naked pointer into the beginning of the Code object. So now the
representation selection always sees a proper tagged value here.

Bug: v8:8015
Change-Id: Ia66e9425d82a0f765e4cbb6e3bed7bcdd9438958
Reviewed-on: https://chromium-review.googlesource.com/1196662Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55524}

src/compiler/simplified-lowering.cc

@@ -1062,12 +1062,6 @@ class RepresentationSelector {
     if (type.IsNone()) {
       return MachineType::None();
     }
-    // TODO(turbofan): Special treatment for ExternalPointer here,
-    // to avoid incompatible truncations. We really need a story
-    // for the JSFunction::entry field.
-    if (type.Is(Type::ExternalPointer())) {
-      return MachineType::Pointer();
-    }
     // Do not distinguish between various Tagged variations.
     if (IsAnyTagged(rep)) {
       return MachineType::AnyTagged();