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

[Matt Arsenault via llvm-commits]

arsenm wrote:

We do need these explicit overrides for some MIR tests, but the description is imprecise:

> This allows for tests where a pass is for example inserting PHI nodes into a function that didn't have any previously.

Passes that insert phis should be clearing the no phis property

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