[llvm] [GlobalMerge] Convert GlobalMerge to a ModulePass (PR #160703)

[via llvm-commits]

paperchalice wrote:

> Going through commit history I was unable to find a justification for why it is written this way, so I will flag to reviewers that it's possible I'm missing something.

This pass must run before `AsmPrinter::doInitialization`.
BTW the comment in AArch64 backend is outdated and misleading.

https://github.com/llvm/llvm-project/pull/160703