Add a regression test for rust-lang#50041

AFAICT the test case never landed alongside the fix for the issue.

src/test/ui/mir/ssa-analysis-regression-50041.rs

@@ -0,0 +1,34 @@
+// build-pass
+// compile-flags: -Z mir-opt-level=3
+
+#![crate_type="lib"]
+#![feature(lang_items)]
+#![no_std]
+
+#[lang = "owned_box"]
+pub struct Box<T: ?Sized>(*mut T);
+
+impl<T: ?Sized> Drop for Box<T> {
+    fn drop(&mut self) {
+    }
+}
+
+#[lang = "box_free"]
+#[inline(always)]
+unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
+    dealloc(ptr)
+}
+
+#[inline(never)]
+fn dealloc<T: ?Sized>(_: *mut T) {
+}
+
+pub struct Foo<T>(T);
+
+pub fn foo(a: Option<Box<Foo<usize>>>) -> usize {
+    let f = match a {
+        None => Foo(0),
+        Some(vec) => *vec,
+    };
+    f.0
+}