NFC: Refactor MIRPrinter into state struct and functions (llvm#140317)

Moves `MIRPRinter` and `MIPrinter` fields into a new `MFPrintState`
struct. Turns methods into independent functions.

- This slightly simplifies the code.
- In the future this will allow to pass the state around to other printing functions
   without the need to expose classes in the public headers.

Author: MatzeB