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

[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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paperchalice wrote:

Some backends, e.g. ARM, use this pass. The legacy version transforms module in `doInitialization` rather than `runOnFunction`. So, add this hook to add global merge and add this pass in front of the pipeline.

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