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

[via llvm-commits]

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@@ -3559,11 +3564,13 @@ class VPlan {
   /// Return the VPIRBasicBlock wrapping the header of the scalar loop.
   VPIRBasicBlock *getScalarHeader() const { return ScalarHeader; }
 
-  /// Return an iterator range over the VPIRBasicBlock wrapping the exit blocks
-  /// of the VPlan, that is leaf nodes except the scalar header. Defined in
-  /// VPlanHCFG, as the definition of the type needs access to the definitions
-  /// of VPBlockShallowTraversalWrapper.
-  auto getExitBlocks();
+  /// Return an ArrayRef containing VPIRBasicBlocks wrapping the exit blocks of
+  /// the original scalar loop.
+  ArrayRef<VPIRBasicBlock *> getExitBlocks() const { return ExitBlocks; }
+
+  /// Return the VPIRBasicBlock corresponding to \p IRBB. \p IRBB must be an
+  /// exit block.
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ayalz wrote:

"must" - if IRBB is not an exit block, an assert is triggered, or null is returned?

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