eliminate the Constant::getVectorElements method. There are better (and

eliminate the Constant::getVectorElements method.  There are better (and
more robust) ways to do what it was doing now.  Also, add static methods
for decoding a ShuffleVector mask.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149028 91177308-0d34-0410-b5e6-96231b3b80d8