llvm-reduce: Make myself maintainer (#133919)

Author: arsenm

llvm/Maintainers.md

@@ -412,6 +412,11 @@ [email protected] (email), [teresajohnson](https://github.com/teresajohnson)
 Aiden Grossman \
 [email protected] (email), [boomanaiden154](https://github.com/boomanaiden154) (Github)
 
+#### llvm-reduce
+
+Matt Arsenault \
+[email protected], [email protected] (email), [arsenm](https://github.com/arsenm) (GitHub)
+
 ### Other
 
 #### Release management