[llvm] [SLP]Support revectorization of the previously vectorized scalars (PR #133091)

[Simon Pilgrim via llvm-commits]

================
@@ -4851,7 +4847,8 @@ class BoUpSLP {
       }
 
       for (const ScheduleEntity *Bundle : ReadyInsts) {
-        assert(Bundle->isReady() && "item in ready list not ready?");
+        assert((Bundle->isReady() || Bundle->isScheduled()) &&
+               "item in ready list not ready?");
         (void)Bundle;
----------------
RKSimon wrote:

```
assert(llvm::all_of(ReadyInsts, [](const ScheduleEntity *Bundle){
      return Bundle->isReady() || Bundle->isScheduled(); })&& "item in ready list not ready?")
```

https://github.com/llvm/llvm-project/pull/133091