[SystemZ] Adding extra extended mnemonics for SystemZ target

This patch consists of the addition of some common additional
extended mnemonics to the SystemZ target.

- These are jnop, jct, jctg, jas, jasl, jxh, jxhg, jxle,
  jxleg, bru, brul, br*, br*l.
- These mnemonics and the instructions they map to are
  defined here, Chapter 4 - Branching with extended
  mnemonic codes.
- Except for jnop (which is a variant of brc 0, label), every
  other mnemonic is marked as a MnemonicAlias since there is
  already a "defined" instruction with the same encoding
  and/or condition mask values.
- brc 0, label doesn't have a defined extended mnemonic, thus
  jnop is defined using as an InstAlias. Furthermore, the
  applyMnemonicAliases function is called in the overridden
  parseInstruction function in SystemZAsmParser.cpp to ensure
  any mnemonic aliases are applied before any further
  processing on the instruction is done.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D92185