[llvm] [RISCV] fix SP recovery in varargs functions (PR #114316)

[via llvm-commits]

dlav-sc wrote:

> Do you plan to precommit the test to show the diff for making the fix? (Your commits here add the test _after_ the fix)

Yep, take a look, please

https://github.com/llvm/llvm-project/pull/114316