add test for better ptr handling in enum niches

tests/run-pass/enum_discriminant_ptr_value.rs

@@ -0,0 +1,9 @@
+// A niche-optimized enum where the discriminant is a pointer value -- relies on ptr-to-int casts in
+// the niche handling code.
+// compile-flags: -Zmiri-disable-stacked-borrows -Zmiri-disable-validation
+
+fn main() {
+    let x = 42;
+    let val: Option<&i32> = unsafe { std::mem::transmute((&x as *const i32).wrapping_offset(2)) };
+    assert!(val.is_some());
+}