[Turbofan] Remove UnallocatedOperand::set_virtual_register.

src/compiler/instruction-selector.cc

Index: src/compiler/instruction-selector.cc
diff --git a/src/compiler/instruction-selector.cc b/src/compiler/instruction-selector.cc
index bafa2c51cd5ae94c74535703ab7e2c13dbbd26d1..8c7ccabcfd1817547040a2e58bf7bdd5b722346a 100644
--- a/src/compiler/instruction-selector.cc
+++ b/src/compiler/instruction-selector.cc
@@ -302,10 +302,11 @@ int InstructionSelector::GetRename(int virtual_register) {
 
 void InstructionSelector::TryRename(InstructionOperand* op) {
   if (!op->IsUnallocated()) return;
-  int vreg = UnallocatedOperand::cast(op)->virtual_register();
+  UnallocatedOperand* unalloc = UnallocatedOperand::cast(op);
+  int vreg = unalloc->virtual_register();
   int rename = GetRename(vreg);
   if (rename != vreg) {
-    UnallocatedOperand::cast(op)->set_virtual_register(rename);
+    *unalloc = UnallocatedOperand(unalloc, rename);
   }
 }