[vim] Fix command already exists on opening multiple mir buffers

[Sisyph]

When using the vim syntax for mir, an error occurs in nvim (unknown if it affects the original vim) when opening multiple .mir buffers. Add '!' when defining HiLink to redefine the command instead of giving an error. Only the changes to mir.vim and llvm.vim are required to fix the error, but from similarity making the change in all the syntax files seems prudent.

To reproduce:
Open an .mir file, for example llvm/test/Codegen/X86/expand-post-ra-pseudo.mir
Open another mir file from within nvim, for example peephole.mir

Error detected while processing function 335[30]..<SNR>43_callback[25]..function 335[30]..<SNR>43_callback:                                                                                                                      
line   23:                                                                                                                                                                                                                       
Vim(command):E174: Command already exists: add ! to replace it: HiLink hi def link <args>   

[Sisyph]

Ping!

[frasercrmck]

Thanks for the PR! Sorry for the delay.

It took me a bit of fiddling to reproduce the error. I had only the llvm.vim and tablegen.vim files copied to my local nvim config, and I didn't see any issue. When I copied over the mir.vim files I started seeing the issue you report. It does seem like plain vim also shows the same problem.

I noticed I wasn't seeing the same issue when opening a tablegen file, so I poked around for a bit. I think the problem with MIR is because it doesn't have delcommand HiLink. If I add that command to mir.vim, the problem also goes away. What do you think about that fix instead?

[Sisyph]

I think the problem with MIR is because it doesn't have delcommand HiLink. If I add that command to mir.vim, the problem also goes away. What do you think about that fix instead?

That makes sense and sounds like a good fix. It worked for me too. Probably better than this one since it is unlikely users would want to call HiLink. Do you want to put that change up, or should I update this one?

[Sisyph]

@frasercrmck I updated this PR with your fix instead. Thanks.

[frasercrmck]

LGTM, thanks!