test: compound assignment LHS index single-evaluation

Pin down that the LHS index expression in a[f()] += v is evaluated
exactly once. Complements viaYul/tuple_evaluation_order.sol.

Author: msoos

test/libsolidity/semanticTests/viaYul/compound_assignment_lhs_side_effect.sol

@@ -0,0 +1,15 @@
+contract C {
+    uint[3] data;
+    uint counter;
+    function nextIdx() internal returns (uint) { counter += 1; return counter - 1; }
+    function f() public returns (uint c, uint a0, uint a1, uint a2) {
+        data[0] = 100;
+        data[1] = 200;
+        data[2] = 300;
+        data[nextIdx()] += 5;
+        c = counter;
+        (a0, a1, a2) = (data[0], data[1], data[2]);
+    }
+}
+// ----
+// f() -> 1, 105, 200, 300