[AMDGPU] Factor out SelectSAddrFI()

[rampitec]

Summary

This is a service function generally useful for selection
of a FI in an SADDR. NFC for now, needed for future patch.

llvm/lib/Target/AMDGPU/AMDGPUISelDAGToDAG.cpp

bool AMDGPUDAGToDAGISel::SelectGlobalSAddr(SDNode *N,
 
   if (!SAddr)
     return false;
 
   Offset = CurDAG->getTargetConstant(ImmOffset, SDLoc(), MVT::i16);
   return true;
 }
 
+static SDValue SelectSAddrFI(SelectionDAG *CurDAG, SDValue SAddr) {
+  if (auto FI = dyn_cast<FrameIndexSDNode>(SAddr)) {
+    SAddr = CurDAG->getTargetFrameIndex(FI->getIndex(), FI->getValueType(0));
+  } else if (SAddr.getOpcode() == ISD::ADD &&
+             isa<FrameIndexSDNode>(SAddr.getOperand(0))) {
+    // Materialize this into a scalar move for scalar address to avoid
+    // readfirstlane.
+    auto FI = cast<FrameIndexSDNode>(SAddr.getOperand(0));
+    SDValue TFI = CurDAG->getTargetFrameIndex(FI->getIndex(),
+                                              FI->getValueType(0));
+    SAddr = SDValue(CurDAG->getMachineNode(AMDGPU::S_ADD_U32, SDLoc(SAddr),
+                                           MVT::i32, TFI, SAddr.getOperand(1)),
+                    0);
+  }
+
+  return SAddr;
+}
+
 // Match (32-bit SGPR base) + sext(imm offset)
 bool AMDGPUDAGToDAGISel::SelectScratchSAddr(SDNode *N,
                                             SDValue Addr,
                                             SDValue &SAddr,
                                             SDValue &Offset) const {
   if (Addr->isDivergent())
     return false;
 
   SAddr = Addr;
   int64_t COffsetVal = 0;
 
   if (CurDAG->isBaseWithConstantOffset(Addr)) {
     COffsetVal = cast<ConstantSDNode>(Addr.getOperand(1))->getSExtValue();
     SAddr = Addr.getOperand(0);
   }
 
-  if (auto FI = dyn_cast<FrameIndexSDNode>(SAddr)) {
-    SAddr = CurDAG->getTargetFrameIndex(FI->getIndex(), FI->getValueType(0));
-  } else if (SAddr.getOpcode() == ISD::ADD &&
-             isa<FrameIndexSDNode>(SAddr.getOperand(0))) {
-    // Materialize this into a scalar move for scalar address to avoid
-    // readfirstlane.
-    auto FI = cast<FrameIndexSDNode>(SAddr.getOperand(0));
-    SDValue TFI = CurDAG->getTargetFrameIndex(FI->getIndex(),
-                                              FI->getValueType(0));
-    SAddr = SDValue(CurDAG->getMachineNode(AMDGPU::S_ADD_U32, SDLoc(SAddr),
-                                           MVT::i32, TFI, SAddr.getOperand(1)),
-                    0);
-  }
+  SAddr = SelectSAddrFI(CurDAG, SAddr);
 
   const SIInstrInfo *TII = Subtarget->getInstrInfo();
 
   if (!TII->isLegalFLATOffset(COffsetVal, AMDGPUAS::PRIVATE_ADDRESS,
                               SIInstrFlags::FlatScratch)) {
     const unsigned NumBits = AMDGPU::getNumFlatOffsetBits(*Subtarget, true);
     // Use signed division by a power of two to truncate towards 0.
     int64_t D = 1LL << (NumBits - 1);