Use unreachable instead of assert(false) to silence MSVC warning

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230045 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

@@ -648,7 +648,7 @@ private:
     case PhiState::Conflict:
       return stateA;
     }
-    assert(false && "only three states!");
+    llvm_unreachable("only three states!");
   }
 };
 }