[ASDAlexander77]

Fixes #450.

x instanceof C with x: C | null, C | D, C | number or C | undefined was always false, under every memory model, even after narrowing (if (e) e instanceof C). mlirGenInstanceOfLogic dispatched on the left operand's type: a class, any, opaque or an interface. A union or an optional fell through to the compile-time comparison of the two types, which emitted the constant false.

Now a union or an optional with a class among its members is asked at run time, through the instance's own vtable (slot 0, .instanceOf), as an any already is:

• an optional, when it has a value;
• C | null, which needs no tag, when the pointer is set;
• a tagged union, when its tag is "class" (every class member's tag is), its payload read as the first class member's pointer. The vtable then decides which class it is, so C | D holding a D gives instanceof D true and instanceof C false.

Anything else it may hold (null, undefined, a number) is an instance of nothing.

Tests

• New tests/00instanceof_union.ts: each shape holding a class and holding something else, a subclass held as its nullable base, a C | D parameter, a narrowed value, and a plain class for comparison. It failed at its first assert before the change. Registered as compile and JIT tests, and in the rc/none corpus list.
• It passes under gc, rc, none and own (AOT and JIT) and with -noopt.
• Full Release ctest: 3444/3445. The one failure is test-compile-gc-defaultlib-collector, the known local LNK2005 against the stale installed default library (fails on main the same way).
• GCC build in WSL; the test passes there under JIT.
• -mm=own corpus: unchanged except the new file, which compiles under own too.

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