[SystemZ] Add support for additional branch extended mnemonics

Added support for extended mnemonics for the following branch instructions and
load/store-on-condition opcodes:

BR, LOCR, LOCGR, LOC, LOCG, STOC, STOCG

Phabricator: http://reviews.llvm.org/D19729

Committing on behalf of Zhan Liau

llvm-svn: 269106