[vim] Clean up HiLink in .mir syntax

[dwoodwor-intel]

Summary

This change adds a missing 'delcommand HiLink'; without this line, vim
prints this error message whenever another IR/MIR file is opened after
opening an MIR file:

E174: Command already exists: add ! to replace it: HiLink hi def link <args>

llvm/utils/vim/syntax/mir.vim

if version >= 508 || !exists("did_c_syn_inits")
   if version < 508
     let did_c_syn_inits = 1
     command -nargs=+ HiLink hi link <args>
   else
     command -nargs=+ HiLink hi def link <args>
   endif
 
   HiLink mirSpecialComment SpecialComment
+
+  delcommand HiLink
 endif
 
 let b:current_syntax = "mir"