Add codegen test for RVO on MaybeUninit

Currently, this only works with `-Cpanic=abort`.

Author: Kobzol

tests/codegen/maybeuninit-rvo.rs

@@ -0,0 +1,19 @@
+// compile-flags: -O -Cpanic=abort
+
+#![crate_type = "lib"]
+
+pub struct Foo([u8; 1000]);
+
+extern "C" {
+    fn init(p: *mut Foo);
+}
+
+pub fn new_from_uninit() -> Foo {
+    // CHECK-LABEL: new_from_uninit
+    // CHECK-NOT: call void @llvm.memcpy.
+    let mut x = std::mem::MaybeUninit::uninit();
+    unsafe {
+        init(x.as_mut_ptr());
+        x.assume_init()
+    }
+}