[CodeGen] Port FreeMachineFunction to new pass manager

[paperchalice]

This pass should be the last machine function pass in pipeline, also ignore PI.runAfterPass(*P, MF, PassPA); to avoid accessing a dangling reference.

[arsenm]

do we somehow block running this with -run-pass? What happens if you manually run this on its own or before other passes?

[paperchalice]

In theory we should never run this pass manually, MachineFunctionPassManager will always create a machine function (by getOrCreateMachineFunction). Or we shouldn't expose this pass , like BitcodeWriterPass.

[aeubanks]

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?

[paperchalice]

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?

May need memory profile here, IIRC this is a memory space trade-off (https://discourse.llvm.org/t/rfc-introducing-classes-for-the-codegen-driven-by-new-pass-manager/55877/19).