[llvm] [CodeGen] Port FreeMachineFunction to new pass manager (PR #79421)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

the intention with FreeMachineFunction is to be able to not have all MFs in memory at once right? however, any module pass in the pipeline breaks that since we'll run passes on every machine function before the module pass

for example, there was a module pass unconditionally added to every codegen pipeline until 2021 in https://reviews.llvm.org/D99707. that makes me wonder, should we just give up on the idea of the FreeMachineFunction pass for simplification, and instead just free all the machine functions at once somehow?

https://github.com/llvm/llvm-project/pull/79421