[turbofan] LoadGlobalParameters::slot_index() should just return an int.

Clang says: warning: 'const' type qualifier on return type has no effect
[-Wignored-qualifiers]

Review URL: https://codereview.chromium.org/1284013003

Cr-Commit-Position: refs/heads/master@{#30123}

src/compiler/js-operator.h

@@ -277,7 +277,7 @@ class LoadGlobalParameters final {
 
   const VectorSlotPair& feedback() const { return feedback_; }
 
-  const int slot_index() const { return slot_index_; }
+  int slot_index() const { return slot_index_; }
 
  private:
   const Unique<Name> name_;