[PowerPC] Don't return false from PPC::isVSLDOIShuffleMask

PPC::isVSLDOIShuffleMask should return -1, not false, when the shuffle
predicate should be false.

Noticed by inspection; no test case (yet).

llvm-svn: 205787

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -929,7 +929,7 @@ bool PPC::isVMRGHShuffleMask(ShuffleVectorSDNode *N, unsigned UnitSize,
 /// amount, otherwise return -1.
 int PPC::isVSLDOIShuffleMask(SDNode *N, bool isUnary) {
   if (N->getValueType(0) != MVT::v16i8)
-    return false;
+    return -1;
 
   ShuffleVectorSDNode *SVOp = cast<ShuffleVectorSDNode>(N);