[IR] Handle constant expressions in containsUndefinedElement()

If the constant is a constant expression, then getAggregateElement()
will return null. Guard against this before calling HasFn().

(cherry picked from commit af382b9)

Author: nikic

llvm/lib/IR/Constants.cpp

@@ -315,9 +315,11 @@ containsUndefinedElement(const Constant *C,
       return false;
 
     for (unsigned i = 0, e = cast<FixedVectorType>(VTy)->getNumElements();
-         i != e; ++i)
-      if (HasFn(C->getAggregateElement(i)))
-        return true;
+         i != e; ++i) {
+      if (Constant *Elem = C->getAggregateElement(i))
+        if (HasFn(Elem))
+          return true;
+    }
   }
 
   return false;

llvm/test/Transforms/InstSimplify/ConstProp/vecreduce.ll

@@ -87,6 +87,15 @@ define i32 @add_poison1() {
   ret i32 %x
 }
 
+define i32 @add_constexpr() {
+; CHECK-LABEL: @add_constexpr(
+; CHECK-NEXT:    [[X:%.*]] = call i32 @llvm.vector.reduce.add.v8i32(<8 x i32> bitcast (<4 x i64> <i64 0, i64 1, i64 2, i64 3> to <8 x i32>))
+; CHECK-NEXT:    ret i32 [[X]]
+;
+  %x = call i32 @llvm.vector.reduce.add.v8i32(<8 x i32> bitcast (<4 x i64> <i64 0, i64 1, i64 2, i64 3> to <8 x i32>))
+  ret i32 %x
+}
+
 define i32 @mul_0() {
 ; CHECK-LABEL: @mul_0(
 ; CHECK-NEXT:    ret i32 0