[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
diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.h b/llvm/lib/Target/SystemZ/SystemZISelLowering.h
index 7a21a47..7d92a73 100644
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.h
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.h
@@ -537,6 +537,7 @@
                                  unsigned UnpackHigh) const;
   SDValue lowerShift(SDValue Op, SelectionDAG &DAG, unsigned ByScalar) const;
 
+  bool canTreatAsByteVector(EVT VT) const;
   SDValue combineExtract(const SDLoc &DL, EVT ElemVT, EVT VecVT, SDValue OrigOp,
                          unsigned Index, DAGCombinerInfo &DCI,
                          bool Force) const;