[llvm] [VPlan] Add VPlan-dump-based test for predication (PR #180794)

[Andrei Elovikov via llvm-commits]

================
@@ -63,7 +64,10 @@ struct VPlanTransforms {
             << "VPlan for loop in '"
             << Plan.getScalarHeader()->getIRBasicBlock()->getParent()->getName()
             << "' after " << PassName << '\n';
-        dbgs() << Plan << '\n';
+        if (VPlanPrintVectorRegionScope && Plan.getVectorLoopRegion())
+          Plan.getVectorLoopRegion()->print(dbgs());
----------------
eas wrote:

No:

```
error: invalid operands to binary expression ('raw_ostream' and 'VPRegionBlock')
```

and the same is true for the `VPValue` hierarchy as well :(

https://github.com/llvm/llvm-project/pull/180794