[llvm] [CodeGen][NPM] Allow nested MF pass managers for -passes (PR #128852)

[Akshat Oke via llvm-commits]

optimisan wrote:

> Why we need this? We already have:
> 
> https://github.com/llvm/llvm-project/blob/d5038b3774485d617e1300cf2f7b98c2460b9042/llvm/include/llvm/IR/PassManager.h#L210-L214
> 
> So the pipeline is still not nested.

I wanted to mean that this is for the cli option `-passes`. Right now this is an error:
```
llc -passes="machine-function(mf1,machine-function(mf2))"
```
Internally, mf2 does get move into mf1's MFPM.

Although this is not vital to have, I wanted to keep it consistent with other pass manager parsers.

https://github.com/llvm/llvm-project/pull/128852