cmd/compile: fix format test

CL 156019 adds some debug output, including printing ssa.relation
as string. Update the map.

Change-Id: I0299d2008d199da10d86e5b47a50385b3a314c68
Reviewed-on: https://go-review.googlesource.com/c/156020
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/fmtmap_test.go

@@ -121,6 +121,7 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/ssa.rbrank %d":              "",
 	"cmd/compile/internal/ssa.regMask %d":             "",
 	"cmd/compile/internal/ssa.register %d":            "",
+	"cmd/compile/internal/ssa.relation %s":            "",
 	"cmd/compile/internal/syntax.Error %q":            "",
 	"cmd/compile/internal/syntax.Expr %#v":            "",
 	"cmd/compile/internal/syntax.Node %T":             "",