[LV][ARM] Add tail-folding tests for MVE. NFC.

D77635 added support to recognise primary induction variables for counting-down
loops. This allows us to fold the scalar tail loop into the main vector body,
which we need for MVE tail-predication. This adds some ARM tail-folding test
cases that we want to support.

This test was extracted from D76838, which implemented a different approach to
reverse and thus find a primary induction variable.