[llvm] [msan] Eliminate non-deterministic behavior in the pass (PR #89831)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

> There is no test, as I failed to reproduce this with opt. My guess is that for reproducer we need to increase fragmentation in the allocator.

Perhaps changing the memory allocator with LD_PRELOAD can more reliably reproduce, but official bots using glibc may not be idiosyncratic enough to reproduce.

https://github.com/llvm/llvm-project/pull/89831