[llvm] [CodeGen] SparseLiveVariables: A liveness analysis pass (PR #167454)

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hiraditya wrote:

> I believe TwoAddressInstructionPass is the one true user of LiveVariables, and only for tied instructions. I'm not sure what PHIElimination is doing with it. I'd probably start by transitioning these to the replacement

makes sense, I'm looking into it.

This PR is ready to be reviewed btw.

https://github.com/llvm/llvm-project/pull/167454