Add codegen test for issue 103840

Author: clubby789

src/test/codegen/issue-103840.rs

@@ -0,0 +1,9 @@
+// compile-flags: -O
+#![crate_type = "lib"]
+
+pub fn foo(t: &mut Vec<usize>) {
+    // CHECK-NOT: __rust_dealloc
+    let mut taken = std::mem::take(t);
+    taken.pop();
+    *t = taken;
+}