[DAG] Use DAGCombiner::SimplifyDemandedBits wrappers with default (all) DemandedElts. NFC.

Don't call TLI.SimplifyDemandedVectorElts directly from every SimplifyDemandedBits call, use the more expressive wrappers instead first.

This reduces the number of places we call TLI.SimplifyDemandedVectorElts and CommitTargetLoweringOpt to make it easier to track.

Part of the work to process DAG nodes in topological order.

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -334,16 +334,11 @@ namespace {
     }
 
     bool SimplifyDemandedBits(SDValue Op, const APInt &DemandedBits) {
-      TargetLowering::TargetLoweringOpt TLO(DAG, LegalTypes, LegalOperations);
-      KnownBits Known;
-      if (!TLI.SimplifyDemandedBits(Op, DemandedBits, Known, TLO, 0, false))
-        return false;
-
-      // Revisit the node.
-      AddToWorklist(Op.getNode());
-
-      CommitTargetLoweringOpt(TLO);
-      return true;
+      EVT VT = Op.getValueType();
+      APInt DemandedElts = VT.isFixedLengthVector()
+                               ? APInt::getAllOnes(VT.getVectorNumElements())
+                               : APInt(1, 1);
+      return SimplifyDemandedBits(Op, DemandedBits, DemandedElts, false);
     }
 
     /// Check the specified vector node value to see if it can be simplified or