interp: fix setting interface objects from operators

This is a follow-up of #1017, generalizing the use of reflect.Set
method to set, and possibly overwrite, the concrete value of
interface objects all accross the implementation of operators.
Previous optimized implementation for non-interface objects is
preserved.

Author: mvertes

_test/interface50.go

@@ -0,0 +1,13 @@
+package main
+
+func main() {
+	a := true
+	var b interface{} = 5
+	println(b.(int))
+	b = a == true
+	println(b.(bool))
+}
+
+// Output:
+// 5
+// true