Index: llvm/include/llvm/Support/TypeSize.h
===================================================================
--- llvm/include/llvm/Support/TypeSize.h
+++ llvm/include/llvm/Support/TypeSize.h
@@ -49,6 +49,33 @@
   bool operator!=(const ElementCount& RHS) const {
     return !(*this == RHS);
   }
+
+  // Returns true if the answer is known at compile time to be >, else we
+  // return false. Known is only set to false if we cannot prove we know the
+  // answer at compile time.
+  bool isGT(const ElementCount &RHS, bool &Known) {
+    bool Result;
+    if ((Scalable || !RHS.Scalable) && Min > RHS.Min)
+      Known = Result = true;
+    else if ((!Scalable || RHS.Scalable) && Min <= RHS.Min) {
+      Known = true;
+      Result = false;
+    } else
+      Known = Result = false;
+    return Result;
+  }
+
+  bool knownGT(const ElementCount &RHS) {
+    bool Known;
+    bool Result = isGT(RHS, Known);
+    assertKnown(Known);
+    return Result;
+  }
+
+private:
+  void assertKnown(bool Known) {
+    assert(Known && "Cannot compare element counts at runtime");
+  }
 };
 
 // This class is used to represent the size of types. If the type is of fixed
Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -437,8 +437,10 @@
     // parts vector has more elements than the value vector, then we have a
     // vector widening case (e.g. <2 x float> -> <4 x float>).  Extract the
     // elements we want.
+    ElementCount PartEC = PartEVT.getVectorElementCount();
+    ElementCount ValueEC = ValueVT.getVectorElementCount();
     if (PartEVT.getVectorElementType() == ValueVT.getVectorElementType()) {
-      assert(PartEVT.getVectorNumElements() > ValueVT.getVectorNumElements() &&
+      assert(PartEC.knownGT(ValueEC) &&
              "Cannot narrow, it would be a lossy transformation");
       return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, ValueVT, Val,
                          DAG.getVectorIdxConstant(0, DL));
@@ -448,8 +450,7 @@
     if (ValueVT.getSizeInBits() == PartEVT.getSizeInBits())
       return DAG.getNode(ISD::BITCAST, DL, ValueVT, Val);
 
-    assert(PartEVT.getVectorNumElements() == ValueVT.getVectorNumElements() &&
-      "Cannot handle this kind of promotion");
+    assert(PartEC == ValueEC && "Cannot handle this kind of promotion");
     // Promoted vector extract
     return DAG.getAnyExtOrTrunc(Val, DL, ValueVT);