[llvm] MachineFunction: Remove null check on TargetRegisterInfo (PR #128480)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

We've not used pure functions here before - are we happy with setting this precedent, and are there any other TargetSubtargetInfo functions that should be made pure?

https://github.com/llvm/llvm-project/pull/128480