Starting to refactor Target to separate out code that's needed to fully describe

Starting to refactor Target to separate out code that's needed to fully describe
target machine from those that are only needed by codegen. The goal is to
sink the essential target description into MC layer so we can start building
MC based tools without needing to link in the entire codegen.

First step is to refactor TargetRegisterInfo. This patch added a base class
MCRegisterInfo which TargetRegisterInfo is derived from. Changed TableGen to
separate register description from the rest of the stuff.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133782 91177308-0d34-0410-b5e6-96231b3b80d8