gc: another shift bug

Fixes #1316.

R=ken2
CC=golang-dev
https://golang.org/cl/3575042
diff --git a/test/const3.go b/test/const3.go
index dd5c889..9bba6ce 100644
--- a/test/const3.go
+++ b/test/const3.go
@@ -26,4 +26,10 @@
 		println("type info didn't propagate in const: got", s)
 		panic("fail")
 	}
+	x := uint(5)
+	y := float64(uint64(1)<<x)	// used to fail to compile
+	if y != 32 {
+		println("wrong y", y)
+		panic("fail")
+	}
 }