ARM64: Introduce TempDoubleRegister as a lithium operand constraint.

src/lithium.h

Index: src/lithium.h
diff --git a/src/lithium.h b/src/lithium.h
index 050b3e5498dac0813c3a79a9e1848dea394e6ec1..d2eb322fd639224f80835a0cbdffbc78cedbfab9 100644
--- a/src/lithium.h
+++ b/src/lithium.h
@@ -81,6 +81,7 @@ class LUnallocated : public LOperand {
     FIXED_REGISTER,
     FIXED_DOUBLE_REGISTER,
     MUST_HAVE_REGISTER,
+    MUST_HAVE_DOUBLE_REGISTER,
     WRITABLE_REGISTER,
     SAME_AS_FIRST_INPUT
   };
@@ -190,6 +191,10 @@ class LUnallocated : public LOperand {
         extended_policy() == WRITABLE_REGISTER ||
         extended_policy() == MUST_HAVE_REGISTER);
   }
+  bool HasDoubleRegisterPolicy() const {
+    return basic_policy() == EXTENDED_POLICY &&
+        extended_policy() == MUST_HAVE_DOUBLE_REGISTER;
+  }
   bool HasSameAsInputPolicy() const {
     return basic_policy() == EXTENDED_POLICY &&
         extended_policy() == SAME_AS_FIRST_INPUT;