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

[Mehdi Amini via llvm-commits]

joker-eph wrote:

For MLIR, here is the code to create and setup an Operation: https://github.com/llvm/llvm-project/blob/main/mlir/lib/IR/Operation.cpp#L114-L120

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