[vim] Fix command already exists on opening multiple mir buffers (#82410)

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>   
```

Author: Sisyph

llvm/utils/vim/syntax/mir.vim

@@ -43,6 +43,8 @@ if version >= 508 || !exists("did_c_syn_inits")
   endif
 
   HiLink mirSpecialComment SpecialComment
+
+  delcommand HiLink
 endif
 
 let b:current_syntax = "mir"