Add mir-opt test for references

Author: 1c3t3a

tests/mir-opt/null_check_references.rs

@@ -0,0 +1,16 @@
+//@ compile-flags: -C debug-assertions
+
+struct Null {
+    a: u32,
+}
+
+fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK-NOT: {{assert.*}}
+    let val: u32 = 42;
+    let val_ref: &u32 = &val;
+    let _access1: &u32 = &*val_ref;
+
+    let val = Null { a: 42 };
+    let _access2: &u32 = &val.a;
+}