Rename `UninhabitedEnumBranching` to `UnreachableEnumBranching`
Per [#120268](https://github.com/rust-lang/rust/pull/120268#discussion_r1517492060), I rename `UninhabitedEnumBranching` to `UnreachableEnumBranching` .
I solved some nits to add some comments.
I adjusted the workaround restrictions. This should be useful for `a <= b` and `if let Some/Ok(v)`. For enum with few variants, `early-tailduplication` should not cause compile time overhead.
r? RalfJung
* The WASI targets deal with the `main` symbol a bit differently than
native so some `codegen` and `assembly` tests have been ignored.
* All `ignore-emscripten` directives have been updated to
`ignore-wasm32` to be more clear that all wasm targets are ignored and
it's not just Emscripten.
* Most `ignore-wasm32-bare` directives are now gone.
* Some ignore directives for wasm were switched to `needs-unwind`
instead.
* Many `ignore-wasm32*` directives are removed as the tests work with
WASI as opposed to `wasm32-unknown-unknown`.
