Give a proper VN to GT_BOX (#15666)

GT_BOX doesn't do anything so it should just get the VN of its sole operand. This allows assertion prop to see that the result of BOX is not null (since the BOX operand is the result of an object allocation which is never null).

Author: mikedn

src/jit/valuenum.cpp

@@ -7046,6 +7046,13 @@ void Compiler::fgValueNumberTree(GenTreePtr tree, bool evalAsgLhsInd)
                     tree->gtVNPair.SetBoth(ValueNumStore::NoVN);
                     break;
 
+                case GT_BOX:
+                    // BOX doesn't do anything at this point, the actual object allocation
+                    // and initialization happens separately (and not numbering BOX correctly
+                    // prevents seeing allocation related assertions through it)
+                    tree->gtVNPair = tree->gtGetOp1()->gtVNPair;
+                    break;
+
                 default:
                     // The default action is to give the node a new, unique VN.
                     tree->gtVNPair.SetBoth(vnStore->VNForExpr(compCurBB, tree->TypeGet()));