[llvm] [RFC] implement convergence control in MIR using SelectionDAG (PR #71785)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm do you see any potential pitfalls in how the glue node is being passed around? The glue is needed only on intrinsics, and there are a couple of FIXME comments about the possibility of some intrinsic having multiple glue nodes. Also, at one point, SelectNodeTo() loses the glue node, so this change first stashes the node, then calls SelectNodeTo(), and then adds the glue node back with MorphNodeTo().

This seems like just a bug in SelectNodeTo. Glue shouldn't just get lost, that kind of defeats the purpose 

https://github.com/llvm/llvm-project/pull/71785