interp: fix type check in constant unary operations

In unary constant operations, the test for unsigned was defeated by
testing for int first, which is true also for unsigned. Make sure that
testing for unsigned precedes testing for int.

Fixes #907.

_test/const20.go

@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+const maxLen = int64(int(^uint(0) >> 1))
+
+func main() {
+	fmt.Println(maxLen)
+}
+
+// Output:
+// 9223372036854775807