DAG: Avoid introducing stack usage in vector->int bitcast int op promotion

 (llvm#125636)

Avoids stack usage in the v5i32 to i160 case for AMDGPU, which appears
in fat pointer lowering.

Author: arsenm

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -566,6 +566,29 @@ SDValue DAGTypeLegalizer::PromoteIntRes_BITCAST(SDNode *N) {
     }
   }
 
+  // TODO: Handle big endian
+  if (!NOutVT.isVector() && InOp.getValueType().isVector() &&
+      DAG.getDataLayout().isLittleEndian()) {
+    // Pad the vector operand with undef and cast to a wider integer.
+    EVT EltVT = InOp.getValueType().getVectorElementType();
+    TypeSize EltSize = EltVT.getSizeInBits();
+    TypeSize OutSize = NOutVT.getSizeInBits();
+
+    if (OutSize.hasKnownScalarFactor(EltSize)) {
+      unsigned NumEltsWithPadding = OutSize.getKnownScalarFactor(EltSize);
+      EVT WideVecVT =
+          EVT::getVectorVT(*DAG.getContext(), EltVT, NumEltsWithPadding);
+
+      if (isTypeLegal(WideVecVT)) {
+        SDValue Inserted = DAG.getNode(ISD::INSERT_SUBVECTOR, dl, WideVecVT,
+                                       DAG.getUNDEF(WideVecVT), InOp,
+                                       DAG.getVectorIdxConstant(0, dl));
+
+        return DAG.getNode(ISD::BITCAST, dl, NOutVT, Inserted);
+      }
+    }
+  }
+
   return DAG.getNode(ISD::ANY_EXTEND, dl, NOutVT,
                      CreateStackStoreLoad(InOp, OutVT));
 }