Fix compilation errors with gcc.

Change-Id: If88d4f639658db2d6d71f5abcad563211138fc4a
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index febe2bc..a2a3cf5 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -1263,6 +1263,8 @@
     case Primitive::kPrimFloat:
       return Location::FpuRegisterLocation(XMM0);
   }
+
+  UNREACHABLE();
 }
 
 Location InvokeDexCallingConventionVisitorX86_64::GetMethodLocation() const {