[X86] Remove stale TODO comment. NFC.

We now handle implicit zero-extension shuffle mask cases.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -40371,8 +40371,6 @@ static SDValue combineExtractWithShuffle(SDNode *N, SelectionDAG &DAG,
       while (Mask.size() > NumSrcElts &&
              canWidenShuffleElements(Mask, WidenedMask))
         Mask = std::move(WidenedMask);
-      // TODO - investigate support for wider shuffle masks with known upper
-      // undef/zero elements for implicit zero-extension.
     }
   }