Adding test for conversion of struct from storage to mapping.

test/libsolidity/semanticTests/structs/struct_storage_to_mapping.sol

@@ -0,0 +1,17 @@
+contract C {
+    struct S {
+        uint a;
+    }
+    S s;
+    mapping (uint => S) m;
+
+    function f() external returns (bool) {
+        s.a = 12;
+        m[1] = s;
+        return m[1].a == 12;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> true