cmd/gc: return canonical Node* from temp

For historical reasons, temp was returning a copy
of the created Node*, not the original Node*.
This meant that if analysis recorded information in the
returned node (for example, n->addrtaken = 1), the
analysis would not show up on the original Node*, the
one kept in fn->dcl and consulted during liveness
bitmap creation.

Correct this, and watch for it when setting addrtaken.

Fixes #7083.

R=khr, dave, minux.ma
CC=golang-codereviews
https://golang.org/cl/51010045

src/cmd/gc/gen.c

@@ -939,5 +939,5 @@ temp(Type *t)
 	n = nod(OXXX, N, N);
 	tempname(n, t);
 	n->sym->def->used = 1;
-	return n;
+	return n->orig;
 }

src/cmd/gc/sinit.c

@@ -468,6 +468,7 @@ staticassign(Node *l, Node *r, NodeList **out)
 			else {
 				a = nod(OXXX, N, N);
 				*a = n1;
+				a->orig = a; // completely separate copy
 				if(!staticassign(a, e->expr, out))
 					*out = list(*out, nod(OAS, a, e->expr));
 			}

src/cmd/gc/typecheck.c

@@ -723,6 +723,8 @@ reswitch:
 		checklvalue(n->left, "take the address of");
 		for(l=n->left; l->op == ODOT; l=l->left)
 			l->addrtaken = 1;
+		if(l->orig != l && l->op == ONAME)
+			fatal("found non-orig name node %N", l);
 		l->addrtaken = 1;
 		defaultlit(&n->left, T);
 		l = n->left;

test/fixedbugs/issue7083.go

+// run
+
+package main
+
+import "runtime/debug"
+
+func f(m map[int]*string, i int) {
+	s := ""
+	m[i] = &s
+}
+
+func main() {
+	debug.SetGCPercent(0)
+	m := map[int]*string{}
+	for i := 0; i < 40; i++ {
+		f(m, i)
+		if len(*m[i]) != 0 {
+			println("bad length", i, m[i], len(*m[i]))
+			panic("bad length")
+		}
+	}
+}