Add test for updating enum discriminant through pointer

tests/codegen/ptr-discriminant-update.rs

@@ -0,0 +1,23 @@
+//@ compile-flags: -O
+//@ min-llvm-version: 19
+
+// Test for #122600
+
+#![crate_type = "lib"]
+
+pub enum State {
+    A([u8; 753]),
+    B([u8; 753]),
+}
+
+// CHECK-LABEL: @update
+#[no_mangle]
+pub unsafe fn update(s: *mut State) {
+    // CHECK-NEXT: start
+    // CHECK-NOT: alloca
+    // CHECK-NOT: memcpy
+    // CHECK-NEXT: store i8
+    // CHECK-NEXT: ret
+    let State::A(v) = s.read() else { std::hint::unreachable_unchecked() };
+    s.write(State::B(v));
+}