[llvm] [AMDGPU] Respect existing glue when lowering convergence tokens (PR #90834)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> > > I guess no nodes actually require this yet. Are you expecting any? Or is this just defensive programming?
> 
> "This" meant "this patch". I was trying to work out what kind of patch this is. Does it fix a bug? Why is there no test case?

This patch merely cements my improved understanding of how glue is supposed to be set on an operation. Callsites already have a glue operand that holds on to the arg nodes. The lowering of control flow tokens at a callsite was adding a second glue operand, which is unexpected. This patch cleans that up by chaining the token with the existing glue operand. In this narrow window inside the AMDGPU backend, this "bug" would have been triggered if those arg operands were actually accessed as glue. But they are not, so nothing broke when I put a second glue operand.

https://github.com/llvm/llvm-project/pull/90834