[llvm] [MIR] Allow overriding isSSA, noPhis, noVRegs in MIR input (PR #108546)

[Matthias Braun via llvm-commits]

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

I guess it's fair enough to add the functionality. Though judging from the tests one can blame some of the problems to the passes:
- If `FinalizeISel` pass is (potentially) creating PHI instructions, then it should probably set the noPhis property to false
- `MachineLICM` deciding `PreRegAlloc = MRI->isSSA();` is probably not great either and maybe we should have two separate pass names for the two use cases or something instead of taking `SSA` as an indicator whether regalloc has happened...
but that may be for another time to clean up...

Anyway probably still a good idea to allow explicitely setting the flags and overriding the computation.

So LGTM

https://github.com/llvm/llvm-project/pull/108546