[SystemZ] Refactor branch and conditional instruction patterns

Rework patterns for branches, call & return instructions,
compare-and-branch, compare-and-trap, and conditional move
instructions.

In particular, simplify creation of patterns for the extended
opcodes of instructions that take a CC mask.

Also, use semantical instruction classes for all the instructions
instead of open-coding them in SystemZInstrInfo.td.

Adds a couple of the basic branch instructions (that are unused
for codegen) for the assembler/disassembler.

llvm-svn: 286263