[llvm-exegesis] Avoid instability from `std::uniform_int_distribution` (#234516)
The standard doesn't require a stable order, and we see active variation
between libstdc++ and libc++ when using this. Follow the pattern of
`llvm::shuffle` and manually clamp with `%` instead.
Also removes an unnecessary `REQUIRES` that gets this to run in
pre-merge CI.
Assisted-by: Antigravity with Gemini