MLIR dead code analysis

[mikeurbach]

I tried out the data flow analysis framework, and also realized the DeadCodeAnalysis was required for pretty much every analysis I wanted to do. So I’m sympathetic to making this easier to get started with, but I’m also curious what making it a “built-in” feature would entail. Maybe rather than built-in in the sense of “always included in DataFlowSolver” is too strong, but we could provide some helper that includes some sensible defaults (e.g. DeadCodeAnalysis and SparseConstantPropagation), which users could extend.