[llvm] [RISCV] Add testcase for return address signing for outlined functions (PR #115079)

[Paul Kirth via llvm-commits]

ilovepi wrote:

Does RISC-V support return address signing? I don't recall a ratified extension that does that.

Additionally, the test code uses `X30` as the return reg, which seems like a mistranslation from Aarch64. I also see that the test uses `auti{a,b}sp`, which are Aarch64 instructions, right?  I think they only pass `FileCheck` because those instructions don't exist for rv64, and they're used in a `CHECK-NOT:` directive.  

Given that we're not testing the signing, and the test IR doesn't seem materially different from the existing cases, do we need this test?

https://github.com/llvm/llvm-project/pull/115079