patch 9.1.0866: filetype: LLVM IR files are not recognized

Problem:  filetype: LLVM IR files are not recognized
Solution: detect '*.ll' files either as lifelines or llvm filetype
          (Wu, Zhenyu)

closes: #15824

Signed-off-by: Wu, Zhenyu <wuzhenyu@ustc.edu>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/dist/ft.vim

@@ -169,6 +169,14 @@ export def FTcls()
   endif
 enddef
 
+export def FTll()
+  if getline(1) =~ ';\|\<source_filename\>\|\<target\>'
+    setf llvm
+  else
+    setf lifelines
+  endif
+enddef
+
 export def FTlpc()
   if exists("g:lpc_syntax_for_c")
     var lnum = 1