[llvm] [VPlan] Construct immutable VPIRBBs for exit blocks at construction(NFC) (PR #128374)

[via llvm-commits]

================
@@ -3426,6 +3426,11 @@ class VPlan {
   /// VPIRBasicBlock wrapping the header of the original scalar loop.
   VPIRBasicBlock *ScalarHeader;
 
+  /// Immutable list of VPIRBasicBlocks wrapping the exit blocks of the original
+  /// scalar loop. Note that some exit blocks may be unreachable, e.g. if the
+  /// scalar epilogue always executes
----------------
ayalz wrote:

```suggestion
  /// scalar epilogue always executes.
```

https://github.com/llvm/llvm-project/pull/128374