[llvm] [PGO] Fix zeroed estimated trip count (PR #167792)

[Mircea Trofin via llvm-commits]

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@@ -912,12 +912,14 @@ llvm::getLoopEstimatedTripCount(Loop *L,
   }
 
   // Return the estimated trip count from metadata unless the metadata is
-  // missing or has no value.
+  // missing or has no value.  Return std::nullopt if it's zero.
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mtrofin wrote:

nit: can you expand why (because it's naively counter-intuitive. I'd just copy and paste the .rst text.)

https://github.com/llvm/llvm-project/pull/167792