[llvm] Don't rely on undefined behavior to store how a `User` object's allocation is laid out (PR #105714)

[Reid Kleckner via llvm-commits]

https://github.com/rnk commented:

Thanks for working on this, it is past time that we did something about it. I remember bumping into this years ago.

I'm not sure this is the best memory layout, but it's going to take some time to think about that.

In the meantime, I started browsing MLIR code to try to figure out how they store the number of operands, presumably without this same UB. I didn't get an answer. @joker-eph , who is the best PoC for that?

https://github.com/llvm/llvm-project/pull/105714