[llvm] [Instrumentation] Avoid repeated hash lookups (NFC) (PR #129988)

[Nikita Popov via llvm-commits]

================
@@ -1517,9 +1517,10 @@ void PGOUseFunc::populateCoverage(IndexedInstrProfReader *PGOReader) {
     CoveredBlocksToProcess.pop();
     for (auto *BB : InverseDependencies[CoveredBlock]) {
       // If CoveredBlock is covered then BB is covered.
-      if (Coverage[BB])
+      auto &Cov = Coverage[BB];
----------------
nikic wrote:

```suggestion
      bool &Cov = Coverage[BB];
```

https://github.com/llvm/llvm-project/pull/129988