[mlir] Add tests for Add trailing cleanup for string type in decode

Signed-off-by: Vladimir Radosavljevic <vr@matterlabs.dev>

Author: vladimirradosavljevic

test/libsolidity/semanticTests/mlir/abi-decode-string-tail-cleanup.sol

@@ -0,0 +1,26 @@
+contract C {
+  function tail_after_decode_memory() public returns (uint256) {
+    bytes memory enc = abi.encode("a");
+    uint256 tail;
+
+    assembly {
+      // Force decode allocation to reuse this dirty chunk.
+      let p := mload(0x40)
+      mstore(add(p, 0x20), not(0))
+      mstore(add(p, 0x40), not(0))
+      mstore(0x40, p)
+    }
+
+    string memory s = abi.decode(enc, (string));
+
+    assembly {
+      tail := mload(add(add(s, 0x20), mload(s)))
+    }
+    return tail;
+  }
+}
+
+// ====
+// compileViaMlir: true
+// ----
+// tail_after_decode_memory() -> 0