[SystemZ] Use named MI sub-operands

Prepare for removing the MemOpsEmitted workaround for symbolic
displacements by letting TableGen know about the offsets of the
displacement sub-operands within the instruction.

There are alternative ways to do this that were tried and rejected:

- Creating encoders and decoders for each possible displacement offset.
  This is too repetitive.

- Use VarLenCodeEmitter [1]. The resulting diff is quite large.

Instead, use the named sub-operand support introduced by commit
a538d1f1 ("[TableGen][CodeEmitterGen] Allow local names for
sub-operands in a operand list.").

Describe instruction encodings in terms of sub-operands instead of
operands (e.g. B, D, L vs BDL) - this also better matches the pictures
from the Principles of Operation. Decompose operands into sub-operands
using the new (bdaddr12only $B1, $D1):$BD1 syntax. Replace the
encoders and the decoders of the operands with these of the
sub-operands.

Since DecodeADDR64BitRegisterClass() is now used for bases and indices,
change it to return NoRegister when decoding 0. This also changes the
disassembly of some instructions, e.g., br %r0 becomes br 0. Since this
better captures the instruction semantics, namely, that the value of
%r0 is not used, keep this change and update the tests.

[1] https://m680x0.github.io/blog/2022/02/varlen-encoder.html

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D155194