[dev.ssa] cmd/compile: detect unbalanced rules

Rules may span multiple lines,
but if we're still unbalanced at the
end of the file, something is wrong.

I write unbalanced rules depressingly often.

Change-Id: Ibd04aa06539e2a0ffef73bb665febf3542fd11f1
Reviewed-on: https://go-review.googlesource.com/12710Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -88,6 +88,9 @@ func genRules(arch arch) {
 		}
 		rule = ""
 	}
+	if unbalanced(rule) {
+		log.Fatalf("unbalanced rule: %v\n", rule)
+	}
 	if err := scanner.Err(); err != nil {
 		log.Fatalf("scanner failed: %v\n", err)
 	}