[llvm] [BPF] Avoid repeated hash lookups (NFC) (PR #131265)

[Nikita Popov via llvm-commits]

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@@ -1026,8 +1026,9 @@ void BTFDebug::constructLineInfo(MCSymbol *Label, const DIFile *File,
   LineInfo.Label = Label;
   LineInfo.FileNameOff = addString(FileName);
   // If file content is not available, let LineOff = 0.
-  if (Line < FileContent[FileName].size())
-    LineInfo.LineOff = addString(FileContent[FileName][Line]);
+  auto &Content = FileContent[FileName];
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nikic wrote:

```suggestion
  const auto &Content = FileContent[FileName];
```

https://github.com/llvm/llvm-project/pull/131265