cmd/compile: keep JMPs around with -N

When -N, make sure we don't drop every instruction from
a block, even ones which would otherwise be empty.
Helps keep line numbers around for debugging, particularly
for break and continue statements (which often compile
down to nothing).

Fixes #14379

Change-Id: I33722c4f0dcd502f146fa48af262ba3a477c959a
Reviewed-on: https://go-review.googlesource.com/19854
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/internal/obj/link.go

@@ -572,6 +572,7 @@ type Link struct {
 	Debugpcln          int32
 	Flag_shared        int32
 	Flag_dynlink       bool
+	Flag_optimize      bool
 	Bso                *Biobuf
 	Pathname           string
 	Windows            int32