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/inst/regs.rs

@@ -178,6 +178,20 @@ pub fn pretty_print_reg(reg: Reg, allocs: &mut AllocationConsumer<'_>) -> String
     show_reg(reg)
 }
 
+pub fn pretty_print_reg_mod(
+    rd: Writable<Reg>,
+    ri: Reg,
+    allocs: &mut AllocationConsumer<'_>,
+) -> String {
+    let output = allocs.next_writable(rd).to_reg();
+    let input = allocs.next(ri);
+    if output == input {
+        show_reg(output)
+    } else {
+        format!("{}<-{}", show_reg(output), show_reg(input))
+    }
+}
+
 pub fn pretty_print_fpr(reg: Reg, allocs: &mut AllocationConsumer<'_>) -> (String, Option<String>) {
     let reg = allocs.next(reg);
     (show_reg(reg), maybe_show_fpr(reg))