Auto merge of #113856 - WaffleLapkin:vtablin', r=oli-obk

Refactor vtable encoding and optimize it for the case of multiple marker traits

This PR does two things
- Refactor `prepare_vtable_segments` (this was motivated by the other change, `prepare_vtable_segments` was quite hard to understand and while trying to edit it I've refactored it)
  - Mostly remove `loop`s labeled `break`s/`continue`s whenever there is a simpler solution
  - Also use `?`
- Make vtable format a bit more efficient wrt to marker traits
  - See the tests for an example

Fixes rust-lang/rust#113840
cc `@crlf0710`

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Review wise it's probably best to review each commit individually, as then it's more clear why the refactoring is correct.

I can split the last two commits (which change behavior) into a separate PR if it makes reviewing easier

Author: bors