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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Who is responsible for clicking the `Resolve Conversation` button? Do I need to do that before merging, will reviewers do that or will they be kept open and I can just go ahead and merge?

It doesn't matter, I prefer if you just do it

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