[llvm] [RISCV] improve `musttail` support (PR #170547)

[Sam Elliott via llvm-commits]

lenary wrote:

We found failures with this too, which we found that applying #185094 fixes (this PR includes a variation of the fix in #185089).

However, I'm more inclined to revert this - there's a lifetime issue in this patch and in #185094 that I think we need to fix - see https://github.com/llvm/llvm-project/pull/185094/changes#r3061266318 . The remaining fixes for musttail in #185094 have a similar lifetime issue, which is why I'm leaning towards a revert rather than fixing forward.

https://github.com/llvm/llvm-project/pull/170547