ARM NEON VST2 assembly parsing and encoding.

ARM NEON VST2 assembly parsing and encoding.

Work in progress. Parsing for non-writeback, single spaced register lists
works now. The rest have the representations better factored, but still
need more to be able to parse properly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146579 91177308-0d34-0410-b5e6-96231b3b80d8