[SystemZ] Add compare-and-branch instructions to MC

This adds MC support for fused compare + indirect branch instructions,
ie. CRB, CGRB, CLRB, CLGRB, CIB, CGIB, CLIB, CLGIB. They aren't actually
generated yet -- this is preparation for their use for conditional
returns in the next iteration of D17339.

Author: koriakin
Differential Revision: http://reviews.llvm.org/D18742

llvm-svn: 265296