[llvm] [CGP] Split off switch cases with 0 and another power-of-2 const. (PR #139321)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

I'm not sure it's a good idea to split up switch lowering like this.

You could maybe argue that that we should most of switch lowering at the LLVM IR level, in general, instead of waiting for SelectionDAG.  But splitting the heuristics so some exist at the IR level, and some exist at the SelectionDAG level, seems like a problem.  Heuristics interact, and when they're split up like this it's hard to tell how they interact.  Like, what the interaction between this, and branch probability optimizations?

https://github.com/llvm/llvm-project/pull/139321