[SelectionDAGBuilder] Make sure DemoteReg ends up in right reg-class.

The virtual register containing the address for returned value on
stack should in the DAG be represented with a CopyFromReg node and not
a Register node. Otherwise, InstrEmitter will not make sure that it
ends up in the right register class for the target instruction.

SystemZ needs this, becuause the reg class for address registers is a
subset of the general 64 bit register class.

test/SystemZ/CodeGen/args-07.ll and args-04.ll updated to run with
-verify-machineinstrs.

Reviewed by Hal Finkel.

llvm-svn: 253461