Add test for 116212.

Author: cjgillot

tests/mir-opt/ssa_unreachable_116212.rs

@@ -0,0 +1,14 @@
+// Regression test for issue #116212.
+
+#![feature(never_type)]
+
+use std::mem::MaybeUninit;
+
+struct Foo {
+    x: u8,
+    y: !,
+}
+
+fn main() {
+    let foo = unsafe { MaybeUninit::<Foo>::uninit().assume_init() };
+}