[DAG][SystemZ] Define unwrapAddress for PCREL_WRAPPER.

[niravd]

Summary

Like with X86, this allows better DAG-level alias analysis and
alignment inference for wrapped addresses.

[uweigand]

LGTM, thanks!

llvm/lib/Target/SystemZ/SystemZISelLowering.h

private:
   SDValue combineFP_ROUND(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue combineFP_EXTEND(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue combineBSWAP(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue combineBR_CCMASK(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue combineSELECT_CCMASK(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue combineGET_CCMASK(SDNode *N, DAGCombinerInfo &DCI) const;
   SDValue combineIntDIVREM(SDNode *N, DAGCombinerInfo &DCI) const;
 
+  SDValue unwrapAddress(SDValue N) const override;
+
   // If the last instruction before MBBI in MBB was some form of COMPARE,
   // try to replace it with a COMPARE AND BRANCH just before MBBI.
   // CCMask and Target are the BRC-like operands for the branch.
   // Return true if the change was made.
   bool convertPrevCompareToBranch(MachineBasicBlock *MBB,
                                   MachineBasicBlock::iterator MBBI,
                                   unsigned CCMask,
                                   MachineBasicBlock *Target) const;

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

SDValue SystemZTargetLowering::combineIntDIVREM(
   // legalization. Therefore we must scalarize this early before Combine
   // 1. For widened vectors, this is already the result of type legalization.
   if (VT.isVector() && isTypeLegal(VT) &&
       DAG.isConstantIntBuildVectorOrConstantInt(N->getOperand(1)))
     return DAG.UnrollVectorOp(N);
   return SDValue();
 }
 
+SDValue SystemZTargetLowering::unwrapAddress(SDValue N) const {
+  if (N->getOpcode() == SystemZISD::PCREL_WRAPPER)
+    return N->getOperand(0);
+  return N;
+}
+
 SDValue SystemZTargetLowering::PerformDAGCombine(SDNode *N,
                                                  DAGCombinerInfo &DCI) const {
   switch(N->getOpcode()) {
   default: break;
   case ISD::ZERO_EXTEND:        return combineZERO_EXTEND(N, DCI);
   case ISD::SIGN_EXTEND:        return combineSIGN_EXTEND(N, DCI);
   case ISD::SIGN_EXTEND_INREG:  return combineSIGN_EXTEND_INREG(N, DCI);
   case SystemZISD::MERGE_HIGH: