AArch64: Fix Managed Register unit tests

Fixes the XZR change introduced by one of the previous patches. It
also adds extra testing for VIXL register integration.

Change-Id: I4935f06726e25829613ef7bb6ac052d82056812c
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>
diff --git a/compiler/utils/arm64/assembler_arm64.h b/compiler/utils/arm64/assembler_arm64.h
index 0220724..8acd1f9 100644
--- a/compiler/utils/arm64/assembler_arm64.h
+++ b/compiler/utils/arm64/assembler_arm64.h
@@ -197,6 +197,8 @@
     CHECK(code < kNumberOfCoreRegisters) << code;
     if (code == SP) {
       return vixl::sp;
+    } else if (code == XZR) {
+      return vixl::xzr;
     }
     return vixl::Register::XRegFromCode(code);
   }
@@ -243,6 +245,9 @@
 
   // List of exception blocks to generate at the end of the code cache.
   std::vector<Arm64Exception*> exception_blocks_;
+
+  // Used for testing.
+  friend class Arm64ManagedRegister_VixlRegisters_Test;
 };
 
 class Arm64Exception {