s390x: update some regalloc metadata to remove use of reg_mod. (#4856)

* s390x: update some regalloc metadata to remove use of `reg_mod`.

This is a step toward ultimately removing modify-operands, which along
with removal of pinned vregs, lets us move to a completely
constraint-based and fully-SSA regalloc input and get some nice
advantages eventually.

There are still a few uses of `mod` operands and pinned vregs remaining,
especially around the "regpair" abstraction. Those proved to be a bit
trickier to update though, so will have to be done separately.

* Review feedback: restore two-arg pretty-print form.

* Review feedback.

cranelift/codegen/src/isa/s390x/abi.rs

@@ -499,6 +499,7 @@ impl ABIMachineSpec for S390xMachineDeps {
             insts.push(Inst::AluRUImm32 {
                 alu_op: ALUOp::AddLogical64,
                 rd: into_reg,
+                ri: into_reg.to_reg(),
                 imm,
             });
         }
@@ -546,12 +547,14 @@ impl ABIMachineSpec for S390xMachineDeps {
             insts.push(Inst::AluRSImm16 {
                 alu_op: ALUOp::Add64,
                 rd: writable_stack_reg(),
+                ri: stack_reg(),
                 imm,
             });
         } else {
             insts.push(Inst::AluRSImm32 {
                 alu_op: ALUOp::Add64,
                 rd: writable_stack_reg(),
+                ri: stack_reg(),
                 imm,
             });
         }