[llvm] [Pass] Support start/stop in instrumentation (PR #70912)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

instead of doing this in the instrumentation, can we do this by not adding the pass to begin with? that seems cleaner.

e.g. `AddMachinePass` can check if we've added some pass and bail if we have/haven't seen the pass so far

https://github.com/llvm/llvm-project/pull/70912