compiler: allow using OP= with struct fields and slice elements

Do it in a generic way, there is no need in restricting to only
variables.
Port of #954.

pkg/compiler/struct_test.go

@@ -134,6 +134,17 @@ var structTestCases = []testCase{
 		}`,
 		big.NewInt(14),
 	},
+	{
+		"increase struct field with +=",
+		`package foo
+		type token struct { x int }
+		func Main() int {
+		t := token{x: 2}
+		t.x += 3
+		return t.x
+		}`,
+		big.NewInt(5),
+	},
 	{
 		"assign a struct field to a struct field",
 		`