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

[Akshat Oke via llvm-commits]

Akshat-Oke wrote:

> 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.

I believe that would require changing how YamlO prints optional fields, unless there is another way to do it here.

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