add regression test for issue #54685

Author: yerke

src/test/ui/function-pointer/function-pointer-comparison-issue-54685.rs

@@ -0,0 +1,31 @@
+// min-llvm-version: 12.0
+// compile-flags: -C opt-level=3
+// run-pass
+
+fn foo(_i: i32) -> i32 {
+    1
+}
+fn bar(_i: i32) -> i32 {
+    1
+}
+
+fn main() {
+    let x: fn(i32) -> i32 = foo;
+    let y: fn(i32) -> i32 = bar;
+
+    let s1;
+    if x == y {
+        s1 = "same".to_string();
+    } else {
+        s1 = format!("{:?}, {:?}", x, y);
+    }
+
+    let s2;
+    if x == y {
+        s2 = "same".to_string();
+    } else {
+        s2 = format!("{:?}, {:?}", x, y);
+    }
+
+    assert_eq!(s1, s2);
+}