[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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205787 91177308-0d34-0410-b5e6-96231b3b80d8