[llvm] [BOLT][BTI] Disassemble PLT entries when processing BTI binaries (PR #169663)

[Maksim Panchenko via llvm-commits]

maksfb wrote:

> Note: there is [an assert checking that PLT functions' Size must be 0](https://github.com/llvm/llvm-project/blob/main/bolt/include/bolt/Core/BinaryFunction.h#L1310). This never triggered during my local testing. I'm not sure why the `Size` (meaning the original size of the function) was set to 0 though.

Since PLT functions are not marked in the symbol table, they were considered pseudo functions with zero size when the support was added. `BinaryFunction::IsPseudo` was added later, and after that I believe it became safe to remove the zero-size assertion.

https://github.com/llvm/llvm-project/pull/169663