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>
diff --git a/runtime/ftplugin/llvm.vim b/runtime/ftplugin/llvm.vim
new file mode 100644
index 0000000..148b12f
--- /dev/null
+++ b/runtime/ftplugin/llvm.vim
@@ -0,0 +1,12 @@
+" Vim filetype plugin file
+" Language:	LLVM IR
+" Last Change:	2024 Oct 22
+" Maintainer:	Wu, Zhenyu <wuzhenyu@ustc.edu>
+
+if exists("b:did_ftplugin") | finish | endif
+let b:did_ftplugin = 1
+
+setl comments=:;
+setl commentstring=;\ %s
+
+let b:undo_ftplugin = "setl commentstring< comments<"