[X86][GlobalISel] Add general-purpose Register Bank

Summary:
[X86][GlobalISel] Add general-purpose Register Bank.
Add trivial  handling of G_ADD legalization .
Add Regestry Bank selection for COPY and G_ADD  instructions

Reviewers: rovka, zvi, ab, t.p.northover, qcolombet

Reviewed By: qcolombet

Subscribers: qcolombet, mgorny, dberris, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D29771

llvm-svn: 294723