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

[Alex Bradbury via llvm-commits]

asb 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`.

Thank you for clarifying why it's written the way it is! I'm seeing failures I don't see locally in the pre-commit CI build, which indicates I got lucky with the order of doInitialization locally.

Have you looked at all on whether it would be possible to remove this dependency?

> BTW the comment in AArch64 backend is outdated and misleading.

The comment can be removed for sure, but it's still a case of an in-tree user of the pass hoping they can have GlobalMerge run to clean up globals emitted by a previous pass. I came across this behaviour when prototyping a pass and looking to use GlobalMerge in the same way.

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