[SystemZ] Copy Access registers and CC with the correct register class.

On SystemZ there are a set of "access registers" that can be copied in and
out of 32-bit GPRs with special instructions. These instructions can only
perform the copy using low 32-bit parts of the 64-bit GPRs. However, the
default register class for 32-bit integers is GRX32, which also contains the
high 32-bit part registers.

In order to never end up with a case of such a COPY into a high reg, this
patch adds a new simple pre-RA pass that selects such COPYs into target
instructions.

This pass also handles COPYs from CC (Condition Code register), and COPYs to
CC can now also be emitted from a high reg in copyPhysReg().

Fixes: https://bugs.llvm.org/show_bug.cgi?id=44254

Review: Ulrich Weigand.

Differential Revision: https://reviews.llvm.org/D75014