Cranelift: Consider rewriting small `br_table`s into conditional branch sequences

I'm seeing a bunch of 3-5 target `br_table`s in `spidermonkey.wasm`, and I suspect that a series of conditional branches might perform better than these small `br_table`s and their indirect jumps. That they are showing up in `spidermonkey.wasm` means that LLVM's Wasm backend is regularly emitting them, so this wouldn't be too overly specific of an optimization.

Could potentially do this in the mid-end or during lowering. Haven't thought too much about it.