Auto merge of #122225 - DianQK:nits-120268, r=cjgillot

 Rename `UninhabitedEnumBranching` to `UnreachableEnumBranching`

Per [#120268](rust-lang/rust#120268 (comment)), 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

Author: bors