cmd/gc: Make sure bools lose idealness when used with logical operators.

Bools from comparisons can be assigned to all bool types, but this idealness would propagate through logical operators when the result should have been lowered to a non-ideal form.

Fixes #3924.

R=golang-dev, remyoudompheng, r, rsc, mtj
CC=golang-dev
https://golang.org/cl/6855061

src/cmd/gc/typecheck.c

@@ -614,7 +614,10 @@ reswitch:
 				n->left = l;
 				n->right = r;
 			}
-		}
+		// non-comparison operators on ideal bools should make them lose their ideal-ness
+		} else if(t == idealbool)
+			t = types[TBOOL];
+
 		if(et == TSTRING) {
 			if(iscmp[n->op]) {
 				n->etype = n->op;

test/fixedbugs/issue3924.go

+// errorcheck
+
+// Copyright 2012 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.
+
+package foo
+
+type mybool bool
+
+var x, y = 1, 2
+var _ mybool = x < y && x < y // ERROR "cannot use"
+var _ mybool = x < y || x < y // ERROR "cannot use"