[llvm] [MIR] Serialize virtual register flags (PR #110228)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

Can you omit printing `, flags: [  ]` in the very common case where it is empty? In fact I do not see any tests where it is _not_ empty.

https://github.com/llvm/llvm-project/pull/110228