Adding semantic tests for bytes to bytesNN conversion.

test/libsolidity/semanticTests/array/bytes_to_fixed_bytes_simple.sol

@@ -0,0 +1,28 @@
+contract C {
+    bytes s = "abcdefghabcdefgh";
+    bytes sLong = "abcdefghabcdefghabcdefghabcdefgh";
+
+    function fromMemory(bytes memory m) public returns (bytes16) {
+        return bytes16(m);
+    }
+    function fromCalldata(bytes calldata c) external returns (bytes16) {
+        return bytes16(c);
+    }
+    function fromStorage() external returns (bytes16) {
+        return bytes16(s);
+    }
+    function fromStorageLong() external returns (bytes32) {
+        return bytes32(sLong);
+    }
+    function fromSlice(bytes calldata c) external returns (bytes8) {
+        return bytes8(c[8:]);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// fromMemory(bytes): 0x20, 16, "abcdefghabcdefgh" -> "abcdefghabcdefgh"
+// fromCalldata(bytes): 0x20, 16, "abcdefghabcdefgh" -> "abcdefghabcdefgh"
+// fromStorage() -> "abcdefghabcdefgh"
+// fromStorageLong() -> "abcdefghabcdefghabcdefghabcdefgh"
+// fromSlice(bytes): 0x20, 16, "abcdefghabcdefgh" -> "abcdefgh"