[SystemZ] Check for presence of vector support in SystemZISelLowering

A test case was found with llvm-stress that caused DAGCombiner to crash
when compiling for an older subtarget without vector support.

SystemZTargetLowering::combineTruncateExtract() should do nothing for older
subtargets.

This check was placed in canTreatAsByteVector(), which also helps in a few
other places.

Review: Ulrich Weigand
llvm-svn: 299763