[Hexagon] Report if changes were made in HvxIdioms pass

This should fix
```
Pass modifies its input and doesn't report it: Hexagon Vector Combine
Pass modifies its input and doesn't report it UNREACHABLE executed at
[...hecks-debian/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1436!
```

llvm/lib/Target/Hexagon/HexagonVectorCombine.cpp

@@ -1342,6 +1342,7 @@ auto HvxIdioms::run() -> bool {
         RecursivelyDeleteTriviallyDeadInstructions(&*It, &HVC.TLI);
         It = StartOver ? B.rbegin()
                        : cast<Instruction>(New)->getReverseIterator();
+        Changed = true;
       }
     }
   }