VSTn instructions have a number of encoding constraints which are not implemented...

VSTn instructions have a number of encoding constraints which are not implemented. I have added these using wrapper methods around the original custom decoder (incidentally - this is a huge poorly written method that should be cleaned up. I have left it as is since the changes would be much to hard to review).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182281 91177308-0d34-0410-b5e6-96231b3b80d8