[LoopVectorize] Permit tail-folding for low trip counts using scalable vectors

When the loop vectoriser encounters a known low trip count it tries
to create a single predicated loop in order to get the benefit of
vectorisation and eliminate the scalar tail. However, until now the
vectoriser prevented the use of scalable vectors in this case due
to concerns in the past about stability. I believe that tail-folded
loops using scalable vectors are now sufficiently well tested that
we can enable this. For the same reason I've also enabled it when
optimising for code size too.

Tests added here:

  Transforms/LoopVectorize/AArch64/sve-low-trip-count.ll
  Transforms/LoopVectorize/AArch64/sve-tail-folding-optsize.ll
  Transforms/LoopVectorize/RISCV/low-trip-count.ll

Differential Revision: https://reviews.llvm.org/D121595

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