ssa: fix newSlice.type & const

cl/_testdata/builtin/in.go

@@ -0,0 +1,11 @@
+package main
+
+var a int64 = 1<<63 - 1
+var b int64 = -1 << 63
+var c uint64 = 1<<64 - 1
+
+func main() {
+	var a = []int{1, 2, 3, 4}
+	_ = len(a)
+	_ = len([]int{1, 2, 3, 4})
+}