[AArch64][NFC] Rewrite aarch64-reassociate-accumulators tests (llvm#142625)

In this patch I've made two changes:

1. The aarch64-reassociate-accumulators-sve.ll test wasn't
representative of loop vectorised code because the pointers were
incrementing by a fixed value and so were only valid when vscale=1. I've
changed this to increment the pointer by the appropriate scalable
amount.
2. I've generated the CHECK lines for both the NEON and SVE test using
filters to make the tests more maintainable.

Author: david-arm