[llvm] [TailDuplicator] Only duplicate the blocks containing computed gotos (PR #114990)

[via llvm-commits]

DianQK wrote:

> > can we simplify the CFG while still keeping the benefit of taildup? Maybe introduce a fake BB into the CFG or something.
> 
> The idea sounds good. Maybe outlined as
> 
> ```
> bb.0:
>   succ: bb.0, bb.1, bb.2
>   indirectbr %0
> bb.1:
>   succ: bb.0, bb.1, bb.2
>   indirectbr %1
> bb.2:
>   succ: bb.0, bb.1, bb.2
>   indirectbr %2
> ```
> 
> =>
> 
> ```
> bb.0:
>   succ: bb.3
>   indirectbr %0
> bb.1:
>   succ: bb.3
>   indirectbr %1
> bb.2:
>   succ: bb.3
>   indirectbr %2
> bb.3:
>   succ: bb.0, bb.1, bb.2
>   %3 = phi(%0, %1, %2)
>   PSEUDO_INDIRECT_BR %3
> ```

I don't fully understand the details of this improvement yet, but it shouldn't block this PR.

So.. ping

https://github.com/llvm/llvm-project/pull/114990