[llvm] [RISCV][CFI] add function epilogue cfi information (PR #110810)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

I did a pretty good go through the tests, and I'm pretty confident in the test output now.

I decided not to have views on the code quality of the implementation itself, partly because I note you have a refactoring follow-up PR (thanks!) and partly because FrameLowering/PEI is already a nightmare at the best of times, and this doesn't seem to make it worse.

Please wait a day or so before landing this PR, in case anyone else wants to comment.

https://github.com/llvm/llvm-project/pull/110810