[LLVM] Volunteer myself as a secondary maintainer for LoopVectorizer

I now have the time and am willing to help out with code reviews,
tidy up tests, clean up code, etc. Over the last few years I've done
quite a lot of vectoriser work, which  includes adding support for
scalable vectors with tail-folding and this year working towards
vectorisation of loops with uncountable early exits. I've also been
actively engaged with reviewing upstream loop vectoriser patches,
and submitting NFC clean-up patches.

Now that we can have a list of maintainers per area I thought perhaps
it's worth formalising things by adding myself as a secondary
maintainer if others are happy?

Author: david-arm

llvm/Maintainers.md

@@ -70,6 +70,8 @@ [email protected] (email), [qcolombet](https://github.com/qcolombet) (G
 
 Florian Hahn \
 [email protected] (email), [fhahn](https://github.com/fhahn) (GitHub)
+David Sherwood \
+[email protected] (email), [david-arm](https://github.com/david-arm) (GitHub)
 
 #### SandboxVectorizer