[NFC][LoopVectorize] Change trip counts for some tests to guarantee a scalar tail

Quite a few vectoriser tests were using a trip count of 1024,
which meant:

1. For fixed-length VFs we would never actually tail-fold, e.g.
see Transforms/LoopVectorize/RISCV/uniform-load-store.ll. This
is because we can prove at compile-time there will never be a
scalar tail.
2. As of D146199 the same optimisation mentioned above will also
apply to scalable VFs too.

I've changed all such trip counts to be 1025 instead.

Differential Revision: https://reviews.llvm.org/D146219