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

[via llvm-commits]

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@@ -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
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ayalz wrote:

All exit blocks all expected to be reachable in the long run. If scalar epilog always executes, the  exit block will be reached only from the scalar epilog (currently unmodelled), w/o a direct edge from middle block to the exit block.

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