[LV] Add test checks when vectorizing loops under opt for size; NFC

Landing this as a separate part of https://reviews.llvm.org/D50480, recording
current behavior more accurately, to clarify subsequent diff ([LV] Vectorizing
loops of arbitrary trip count without remainder under opt for size).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344606 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed