add codegen test for rust-lang#120440

Author: tesuji

tests/codegen/slice-pointer-nonnull-unwrap.rs

@@ -0,0 +1,19 @@
+//@ compile-flags: -O
+//@ min-llvm-version: 18
+#![crate_type = "lib"]
+
+use std::ptr::NonNull;
+
+// CHECK-LABEL: @slice_ptr_len_1
+// CHECK: {{.*}}:
+// CHECK-NEXT: ret i64 %ptr.1
+#[no_mangle]
+pub fn slice_ptr_len_1(ptr: *const [u8]) -> usize {
+  let ptr = ptr.cast_mut();
+  if let Some(ptr) = NonNull::new(ptr) {
+    ptr.len()
+  } else {
+    // We know ptr is null, so we know ptr.wrapping_byte_add(1) is not null.
+    NonNull::new(ptr.wrapping_byte_add(1)).unwrap().len()
+  }
+}