[llvm] [SelectionDAG] Fix a false assumption that there will always be a valid integer type corresponding to a vector type (PR #96658)

[Simon Pilgrim via llvm-commits]

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@@ -1468,13 +1468,31 @@ SDValue SelectionDAG::getZExtOrTrunc(SDValue Op, const SDLoc &DL, EVT VT) {
 }
 
 SDValue SelectionDAG::getBitcastedAnyExtOrTrunc(SDValue Op, const SDLoc &DL,
-                                                 EVT VT) {
+                                                EVT VT) {
   assert(!VT.isVector());
   auto Type = Op.getValueType();
   SDValue DestOp;
   if (Type == VT)
     return Op;
   auto Size = Op.getValueSizeInBits();
+  auto IntTy = MVT::getIntegerVT(Size);
+
+  if (!IntTy.isValid()) {
+    // We assume integers of "weird" size have already been legalized here.
+    assert(Type.isVector());
+    unsigned NumElements = Type.getVectorNumElements();
+    unsigned ExtSize = VT.getScalarSizeInBits();
+    EVT ElementType = Type.getVectorElementType();
+    unsigned ExtNumElements = ExtSize / ElementType.getScalarSizeInBits();
+    assert(NumElements < ExtNumElements);
+    MVT ExtType = MVT::getVectorVT(ElementType.getSimpleVT(), ExtNumElements);
+    SmallVector<SDValue, 4> Ops(ExtNumElements, getUNDEF(ElementType));
+    SDValue ExtVec = getNode(ISD::BUILD_VECTOR, DL, ExtType, Ops);
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RKSimon wrote:

Why not getUNDEF(ExtType)? If you do need to handle implicit truncation inside the BUILD_VECTOR you can use getSplat / getSplatBuildVector instead.

https://github.com/llvm/llvm-project/pull/96658