[llvm] machinebackend pass test (PR #122487)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@Ryan-hub-bit its better to only raise a PR once a patch branch is complete and ready for review - although if you're after early feedback its fine to make this a draft (as I've just done).

https://github.com/llvm/llvm-project/pull/122487