DAGCombine should not aggressively fold SEXT(VSETCC(...)) into a wider VSETCC without first checking the target's vector boolean contents.
This exposed an issue with PowerPC AltiVec where it appears it was setting the wrong vector boolean contents.  The included change
fixes the PowerPC tests, and was OK'd by Hal.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180129 91177308-0d34-0410-b5e6-96231b3b80d8