[llvm] [llvm-exegesis] Fix preservation of RDI in subprocess mode (PR #72458)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

@ondrasej ping since you're not in the LLVM org and I can't add you as a reviewer.

This is the first of probably a couple fixes related to register preservation in `llvm-exegesis`. I'll slowly add test coverage to the `subprocess-preserved-register.s` file as I get to them, this was just one of the easiest ones to fix, so I figured I would get it up first so that it can get reviewed.

https://github.com/llvm/llvm-project/pull/72458