[llvm] [SelectionDAG][RISCV] Account for samesign in preferred export extensions (PR #226013)

[via llvm-commits]

imkiva wrote:

> Please can you restructure this PR:
> 
> * the first commit should add the tests only
> * the second commit should add the codegen changes, and update the tests
> 
> This makes it far easier to see and review the improvements you're making.

Thanks, I reorganized this PR accroding to your suggestion. Looks much better now~

https://github.com/llvm/llvm-project/pull/226013