[vim] Highlight most common MIR syntax not in LLVM IR

[porglezomp]

Summary

This adds highlighting for MIR instruction opcodes, physical registers,
and MIR types.

[dblaikie]

Don't have much context/knowledge of vim scripts, but even something half working seems fine by me.

llvm/utils/vim/syntax/machine-ir.vim

 " Vim syntax file
 " Language:   mir
 " Maintainer: The LLVM team, http://llvm.org/
 " Version:      $Revision$
 
-" FIXME: MIR doesn't actually match LLVM IR. Reimplement this.
+syn case match
+
+" FIXME: MIR doesn't actually match LLVM IR. Stop including it all as a
+" fallback once enough is implemented.
 " See the MIR LangRef: https://llvm.org/docs/MIRLangRef.html
 unlet b:current_syntax  " Unlet so that the LLVM syntax will load
 runtime! syntax/llvm.vim
 unlet b:current_syntax
+
+syn match   mirType /\<[sp]\d\+\>/
+
+" Opcodes. Matching instead of listing them because individual targets can add
+" these. FIXME: Maybe use some more context to make this more accurate?
+syn match   mirStatement /\<[A-Z][A-Za-z0-9_]*\>/
+
+syn match   mirPReg /$[-a-zA-Z$._][-a-zA-Z$._0-9]*/
+
+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 mirType Type
+  HiLink mirStatement Statement
+  HiLink mirPReg Identifier
+
+  delcommand HiLink
+endif
+
 let b:current_syntax = "mir"