[SystemZ] Call tryAddingSymbolicOperand in the disassembler

Use the tryAddingSymbolicOperand callback to attempt to present immediate
values in symbolic form when disassembling.  This is currently only used
for PC-relative immediates (which are most likely to be symbolic in the
SystemZ ISA).  Add new DecodeMethod types to allow distinguishing between
branch and non-branch instructions.

llvm-svn: 266469