[InstCombine] SimplifyDemandedUseBits - add TODO to remove shl node if we only demand known sign bits of the shift source

Similar to what we already perform for ashr/lshr

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -563,6 +563,9 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
                                                     DemandedMask, Known))
             return R;
 
+      // TODO: If we only want bits that already match the signbit then we don't
+      // need to shift.
+
       uint64_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
       APInt DemandedMaskIn(DemandedMask.lshr(ShiftAmt));