ARM: stop passing unused values up the TableGen hierarchy.

It's bad enough that I have to look up 5 different levels of TableGen class
definitions to work out what bits go where in a simple NEON instruction anyway,
without having to keep track of umpteen unused parameters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207420 91177308-0d34-0410-b5e6-96231b3b80d8