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

[Matt Arsenault via llvm-commits]

arsenm 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 I don't know of.

For the other fields, I thought this changed based on the -simplify-mir flag. (Also, -simplify-mir should really be the default) 

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