[NFC] Add tests for scalable vectorization of loops with in-order reductions

D98435 added support for in-order reductions and included tests for fixed-width
vectorization with the -enable-strict-reductions flag.

This patch adds similar tests to verify support for scalable vectorization of loops
with in-order reductions.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D100385