[llvm] [CodeGen] Port GlobalMerge to new pass manager (PR #77474)

[Matt Arsenault via llvm-commits]

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@@ -290,6 +291,9 @@ template <typename DerivedT> class CodeGenPassBuilder {
                                    inconvertibleErrorCode());
   }
 
+  /// Target can override this to add GlobalMergePass before all IR passes.
+  void addGlobalMergePass(AddIRPass &) const {}
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arsenm wrote:

Something doesn't make sense. Why is this a function pass, that only changes the module and does all the work in doInitalization? Should it be converted to a ModulePass?

https://github.com/llvm/llvm-project/pull/77474