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

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/697073002

Cr-Commit-Position: refs/heads/master@{#25066}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25066 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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));
 }