interp: fix operators working on integer constants

Always attempt to obtain an integer constant value for operators
expecting so. It allows to use '/' in integer constant defintions,
instead of default big.Rat.

Fixes #1005

Author: mvertes

_test/const22.go

@@ -0,0 +1,15 @@
+package main
+
+const (
+	numDec uint8 = (1 << iota) / 2
+	numHex
+	numOct
+	numFloat
+)
+
+func main() {
+	println(13 & (numHex | numOct))
+}
+
+// Output:
+// 1