cmd/compile: fix ssa/debug_test.go reference file

Behavior improved, file expects old worse behavior.
Update file to expect newer, better output.

Fixes #30912.

Change-Id: I410bdaacdb77a4620656753c8c0dc2d4d5378985
Reviewed-on: https://go-review.googlesource.com/c/go/+/168377
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

src/cmd/compile/internal/ssa/testdata/i22558.gdb-dbg.nexts

@@ -2,3 +2,10 @@
 19:	func test(t *thing, u *thing) {
 20:		if t.next != nil {
 23:		fmt.Fprintf(os.Stderr, "%s\n", t.name)
+24:		u.self = u
+25:		t.self = t
+26:		t.next = u
+27:		for _, p := range t.stuff {
+28:			if isFoo(t, p) {
+29:				return
+44:	}