[LLVM] Volunteer myself as a secondary maintainer for LoopVectorizer (#120704)

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?

Not entirely sure who to add as reviewers on this PR though!

Author: david-arm

llvm/Maintainers.md

@@ -87,7 +87,9 @@ [email protected] (email), [qcolombet](https://github.com/qcolombet) (G
 #### LoopVectorize
 
 Florian Hahn \
-[email protected] (email), [fhahn](https://github.com/fhahn) (GitHub)
+[email protected] (email), [fhahn](https://github.com/fhahn) (GitHub) \
+David Sherwood \
+[email protected] (email), [david-arm](https://github.com/david-arm) (GitHub)
 
 #### MemorySSA