[llvm] [SandboxIR] Preserve the order of switch cases after revert. (PR #115577)

[Jorge Gorbe Moya via llvm-commits]

slackito wrote:

> This solution is still implementation dependent given the lack of an addCase() API call with index. Not sure if I prefer this one over the previous one, given that the previous one was a constant-time operation.

I'm not super happy about it either, but I think this one is more robust against potential (if unlikely) `removeCase` changes.

If this turns out to be performance-sensitive we can go back to the other solution, and maybe open an RFC to document the existing behavior so we can rely on it.

https://github.com/llvm/llvm-project/pull/115577