[TableGen] Add a way of getting the number of generic opcodes without including modular CodeGen headers.

This is a bit of a hack, but removes a cycle that broke modular builds
of LLVM. Of course the cycle is still there in form of a dependency
on the .def file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323383 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed