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

[Sam Elliott via llvm-commits]

lenary 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.


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