gc: Nicer errors before miscompiling.

This fixes issue 2444.

A big cleanup of all 31/32bit size boundaries i'll leave for another cl though.  (see also issue 1700).

R=rsc
CC=golang-dev
https://golang.org/cl/5484058

test/fixedbugs/bug385_32.go

@@ -0,0 +1,14 @@
+// [ $O == 6 ] || errchk $G -e $D/$F.go
+
+// Copyright 2011 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 2444
+
+package main
+func main() {
+	var arr [1000200030]int   // ERROR "type .* too large"
+	arr_bkup := arr
+	_ = arr_bkup
+}