[AArch64][NFC] Simplify loop vectoriser tail-folding tests

I've simplified all of the SVE vectoriser tail-folding tests to
only care about testing the flag:

  -prefer-predicate-over-epiloge=predicate-else-scalar-epilogue

In practice we always want to fall back on unpredicated vector
loops if tail-folding is not possible.

Differential Revision: https://reviews.llvm.org/D129843