[SystemZ] Add check VT.isSimple() in canTreateAsByteVector()

Since BB-vectorizer can produce vectors of for example 3 elements,
this check is needed.

Review: Ulrich Weigand

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297136 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed