[turbofan] Enforce allocation of register for imull/mull.

src/compiler/x64/instruction-selector-x64.cc

Index: src/compiler/x64/instruction-selector-x64.cc
diff --git a/src/compiler/x64/instruction-selector-x64.cc b/src/compiler/x64/instruction-selector-x64.cc
index cd039ed2882299f51798fdf7530db84e22d8d7f0..f927501c6531e45ecfd6024e5545fca253846264 100644
--- a/src/compiler/x64/instruction-selector-x64.cc
+++ b/src/compiler/x64/instruction-selector-x64.cc
@@ -414,8 +414,10 @@ void VisitMulHigh(InstructionSelector* selector, Node* node,
   if (selector->IsLive(left) && !selector->IsLive(right)) {
     std::swap(left, right);
   }
+  // TODO(turbofan): We use UseUniqueRegister here to improve register
+  // allocation.
   selector->Emit(opcode, g.DefineAsFixed(node, rdx), g.UseFixed(left, rax),
-                 g.UseUnique(right));
+                 g.UseUniqueRegister(right));
 }