[llvm] [SCCPSolver] Make getMRVFunctionsTracked return a reference (NFC) (PR #140851)

[Kazu Hirata via llvm-commits]

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@@ -147,7 +147,7 @@ class SCCPSolver {
 
   /// getMRVFunctionsTracked - Get the set of functions which return multiple
   /// values tracked by the pass.
-  const SmallPtrSet<Function *, 16> getMRVFunctionsTracked();
+  const SmallPtrSet<Function *, 16> &getMRVFunctionsTracked();
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kazutakahirata wrote:

@fhahn Could you elaborate on what "marked as onsets" mean?

https://github.com/llvm/llvm-project/pull/140851