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

[Nikita Popov via llvm-commits]

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@@ -1563,14 +1564,15 @@ void PGOUseFunc::populateCoverage(IndexedInstrProfReader *PGOReader) {
     // successors, e.g., when a block calls a function that may call exit(). In
     // those cases, BFI could find its successor to be covered while BCI could
     // find its successor to be dead.
-    if (Coverage[&BB] == IsBlockDead(BB).value_or(false)) {
+    auto &Cov = Coverage[&BB];
----------------
nikic wrote:

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

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