Enable -Wunreachable-code

Caught bugs in DeoptimizeStackVisitor and assemble_x86 SIB encoding.
Add UNREACHABLE macro to document code expected to be unreachable.
Bug: 17731047

Change-Id: I2e363fe5b38a1246354d98be18c902a6031c0b9e
diff --git a/runtime/mirror/object.cc b/runtime/mirror/object.cc
index a1177d6..57069ab 100644
--- a/runtime/mirror/object.cc
+++ b/runtime/mirror/object.cc
@@ -187,8 +187,7 @@
       }
     }
   }
-  LOG(FATAL) << "Unreachable";
-  return 0;
+  UNREACHABLE();
 }
 
 void Object::CheckFieldAssignmentImpl(MemberOffset field_offset, Object* new_value) {