cmd/gc: use a register to checknil constants.

Fixes #7346.

LGTM=rsc
R=rsc, iant, khr
CC=golang-codereviews
https://golang.org/cl/69050044

src/cmd/gc/pgen.c

@@ -476,7 +476,7 @@ cgen_checknil(Node *n)
 		dump("checknil", n);
 		fatal("bad checknil");
 	}
-	if((thechar == '5' && n->op != OREGISTER) || !n->addable) {
+	if((thechar == '5' && n->op != OREGISTER) || !n->addable || n->op == OLITERAL) {
 		regalloc(&reg, types[tptr], n);
 		cgen(n, &reg);
 		gins(ACHECKNIL, &reg, N);

test/fixedbugs/issue7346.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.
+
+// issue 7346 : internal error "doasm" error due to checknil
+// of a nil literal.
+
+package main
+
+func main() {
+	_ = *(*int)(nil)
+}