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

[via llvm-commits]

================
@@ -9140,6 +9140,10 @@ collectUsersInExitBlocks(Loop *OrigLoop, VPRecipeBuilder &Builder,
                          VPlan &Plan) {
   SetVector<VPIRInstruction *> ExitUsersToFix;
   for (VPIRBasicBlock *ExitVPBB : Plan.getExitBlocks()) {
+    // Nothing to do for unreachable exit blocks.
----------------
ayalz wrote:

All exit blocks are expected to be reachable, in the long run.
The exit blocks of Plan include only exit.block and optionally latch.exit, as depicted in
https://llvm.org/docs/Vectorizers.html#epilogue-vectorization and https://llvm.org/docs/Vectorizers.html#early-exit-vectorization, where the scalar header is expected to eventually reach the above exit blocks.

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