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/slice_test.go

@@ -33,6 +33,16 @@ var sliceTestCases = []testCase{
 		`,
 		big.NewInt(42),
 	},
+	{
+		"increase slice element with +=",
+		`package foo
+		func Main() int {
+			a := []int{1, 2, 3}
+			a[1] += 40
+			return a[1]
+		}`,
+		big.NewInt(42),
+	},
 	{
 		"complex test",
 		`