[SystemZ] Model access registers as LLVM registers

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286283 91177308-0d34-0410-b5e6-96231b3b80d8