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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

We need this to avoid artificially breaking these properties in tests, but conflicts should be errors. We don't want to pollute mir tests with wrong markers 

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