[NFC][LoopVectorize] Explicitly disable tail-folding on some SVE tests

This patch is in preparation for enabling vectorisation with tail-folding
by default for SVE targets. Once we do that many existing tests will
break that depend upon having normal unpredicated vector loops. For
all such tests I have added the flag:

  -prefer-predicate-over-epilogue=scalar-epilogue

Differential Revision: https://reviews.llvm.org/D129137