cmd/gc: fix duplicate map key check

Do not compare nil and true.

Fixes #7996.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/91470043

src/cmd/gc/typecheck.c

@@ -2415,23 +2415,19 @@ keydup(Node *n, Node *hash[], ulong nhash)
 	for(a=hash[h]; a!=N; a=a->ntest) {
 		cmp.op = OEQ;
 		cmp.left = n;
+		b = 0;
 		if(a->op == OCONVIFACE && orign->op == OCONVIFACE) {
-			if(a->left->type == n->type) {
+			if(eqtype(a->left->type, n->type)) {
 				cmp.right = a->left;
 				evconst(&cmp);
 				b = cmp.val.u.bval;
 			}
-			else {
-				b = 0;
-			}
-		}
-		else {
+		} else if(eqtype(a->type, n->type)) {
 			cmp.right = a;
 			evconst(&cmp);
 			b = cmp.val.u.bval;
 		}
 		if(b) {
-			// too lazy to print the literal
 			yyerror("duplicate key %N in map literal", n);
 			return;
 		}

test/fixedbugs/issue7996.go

+// compile
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// /tmp/x.go:5: illegal constant expression: bool == interface {}
+
+package p
+
+var m = map[interface{}]struct{}{
+	nil:  {},
+	true: {},
+}