When using the vim syntax for mir, an error occurs in nvim when opening
multiple .mir buffers. delcommand HiLink in the mir syntax file to avoid
the issue.
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>
```
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