[llvm] [NFC][KeyInstr] Add (LLVM_)EXPERIMENTAL_KEY_INSTRUCTIONS (cmake/)definition (PR #131344)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

I hope it's not too pushy in landing this, as it's not clear the RFC for key instructions is "accepted". This can easily be reverted; I'm just trying to set up the testing path.

https://github.com/llvm/llvm-project/pull/131344