Add a regression test for #76281

This has been fixed between 1.47.0-nightly (663d2f5 2020-08-22) and 1.47.0-nightly (5180f3d 2020-08-23).

src/test/ui/wasm/wasm-hang-issue-76281.rs

@@ -0,0 +1,12 @@
+// only-wasm32
+// compile-flags: -C opt-level=2
+// build-pass
+
+// Regression test for #76281.
+// This seems like an issue related to LLVM rather than
+// libs-impl so place here.
+
+fn main() {
+    let mut v: Vec<&()> = Vec::new();
+    v.sort_by_key(|&r| r as *const ());
+}