liblink: code for debugging bad returns

Disabled by default, but invaluable when you need it.

Change-Id: If4a75d11d14f70b6840d339aaec4b940dc406493
Reviewed-on: https://go-review.googlesource.com/2012Reviewed-by: Russ Cox <rsc@golang.org>

src/liblink/obj9.c

@@ -619,6 +619,22 @@ addstacksplit(Link *ctxt, LSym *cursym)
 			p->link = q;
 			p = q;
 
+			if(0) {
+				// Debug bad returns
+				q = ctxt->arch->prg();
+				q->as = AMOVD;
+				q->lineno = p->lineno;
+				q->from.type = D_OREG;
+				q->from.offset = 0;
+				q->from.reg = REGTMP;
+				q->to.type = D_REG;
+				q->to.reg = REGTMP;
+
+				q->link = p->link;
+				p->link = q;
+				p = q;
+			}
+
 			if(autosize) {
 				q = ctxt->arch->prg();
 				q->as = AADD;