[SystemZ] Model access registers as LLVM registers

Add the 16 access registers as LLVM registers.  This allows removing
a lot of special cases in the assembler and disassembler where we
were handling access registers; this can all just use the generic
register code now.

Also add a bunch of instructions to operate on access registers,
for assembler/disassembler use only.  No change in code generation
intended.

llvm-svn: 286283