Rework arm64 register codes and fix Arm64ManagedRegister tests.

Change-Id: I81ce3bc8a212c9c35be3a41b182ada87b32391ec
diff --git a/compiler/utils/arm64/assembler_arm64.h b/compiler/utils/arm64/assembler_arm64.h
index ef83334..cf831f8 100644
--- a/compiler/utils/arm64/assembler_arm64.h
+++ b/compiler/utils/arm64/assembler_arm64.h
@@ -183,6 +183,11 @@
   }
 
   static vixl::Register reg_w(int code) {
+    if (code == WSP) {
+      return vixl::wsp;
+    } else if (code == WZR) {
+      return vixl::wzr;
+    }
     return vixl::Register::WRegFromCode(code);
   }