[Turbofan] Remove UnallocatedOperand::set_virtual_register.

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index 3f9d5bfcefacc64a9ba47c07ec1aeb5e97802124..d38df078e13636a6ba75b6635aae1cf7bb951217 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -1777,7 +1777,8 @@ void ConstraintBuilder::MeetConstraintsBefore(int instr_index) {
     int output_vreg = second_output->virtual_register();
     int input_vreg = cur_input->virtual_register();
     UnallocatedOperand input_copy(UnallocatedOperand::ANY, input_vreg);
-    cur_input->set_virtual_register(second_output->virtual_register());
+    *cur_input =
+        UnallocatedOperand(*cur_input, second_output->virtual_register());
     MoveOperands* gap_move = data()->AddGapMove(instr_index, Instruction::END,
                                                 input_copy, *cur_input);
     if (code()->IsReference(input_vreg) && !code()->IsReference(output_vreg)) {