Sema: Fix switch loop OPV cond lowering

[Justus2308]

Resolves #21550
Resolves #24126
Resolves #21772

Special-cases runtime loops with a condition with one possible value and lowers them to a regular loop instead of a loop_switch_br construct. Also covers a switch loop with a single else prong, see comment below.
The implementation is basically a simplified version of analyzeSwitchRuntimeBlock and the code following it in zirSwitchBlock that makes lots of assumptions and omits any value corruption safety checks or prong branch hints.

This patch takes advantage of the symmetry between loop_switch_br-switch_dispatch and loop-repeat and relies on backends being able to handle a loop targeted by multiple repeats. This pattern should be possible but has never been emitted by Sema until now.
The added tests skip the self-hosted spirv backend because it doesn't support this pattern yet.

Also fixes a typo in Liveness.

[Justus2308]

I realized that this patch also works for the general case of a switch loop with a single non-inlined else/_ prong and no other prongs so I added that. It's a free optimization because we already check for that case anyway for some safety checks in zirSwitchBlock.

[Justus2308]

This is kinda blocked by my own PR (#24381) for now lol, I'll continue working on it once that's merged
I might expand on this to also handle switch (cond) { else => {} } once that's happened, but that will be a separate PR

[Justus2308]

Ok I believe I've found a much better solution to this problem now that can reuse most of the code that's already there and handles switch (cond) { else => {} }. It does include some std.meta abuse (nothing too bad though), I'm open to suggestions for nicer alternatives :)

[Justus2308]

I managed to get rid of the std.meta usage. Sorry for reworking this PR so much, I promise this is the last major change!

[andrewrk]

Sorry for reworking this PR so much

that's no problem, I for one appreciate when someone takes the time to get it right.