[llvm] [UniformityAnalysis] Add CallbackVH to keep UniformValues consistent on value deletion (PR #187658)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> Can you please provide more context on where this is going to be used? It is possible to explicitly remove values from the analysis in relevant places? I think we've had enough bad experience with this "let the analysis magically fix itself" approach that I would discourage adding it to something new, especially if it is as rarely used as UniformityAnalysis.

It's more a case of "UA _must_ be immediately updated if a value is deleted" for correctness. `Value*` pointers tracked in the UA must always be those that were added after determining their uniformity. So if a dangling pointer got reused, it would mean that the new `Value` is now considered uniform without any due diligence.

https://github.com/llvm/llvm-project/pull/187658