exp/locale/collate: avoid 16-bit math

There's no need for the 16-bit arithmetic here,
and it tickles a long-standing compiler bug.
Fix the exp code not to use 16-bit math and
create an explicit test for the compiler bug.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6256048

src/pkg/exp/locale/collate/colelem.go

@@ -102,7 +102,7 @@ const (
 )
 
 func splitContractIndex(ce colElem) (index, n, offset int) {
-	h := uint16(ce)
+	h := ce & 0xffff
 	return int(h >> maxNBits), int(h & (1<<maxNBits - 1)), int(ce>>16) & (1<<maxContractOffsetBits - 1)
 }
 

test/bugs/bug440.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+// # switch above to 'run' when bug gets fixed.
+// # right now it only breaks on 8g
+
+// Test for 8g register move bug.  The optimizer gets confused
+// about 16- vs 32-bit moves during splitContractIndex.
+
+package main
+
+func main() {
+	const c = 0x12345678
+	index, n, offset := splitContractIndex(c)
+	if index != int((c&0xffff)>>5) || n != int(c & (1<<5-1)) || offset != (c>>16)&(1<<14-1) {
+		println("BUG", index, n, offset)
+	}
+}
+
+func splitContractIndex(ce uint32) (index, n, offset int) {
+	h := uint16(ce)
+	return int(h >> 5), int(h & (1<<5 - 1)), int(ce>>16) & (1<<14 - 1)
+}