Reflect control flow effects of optional chains in equality checks

[ahejlsberg]

With this PR we properly reflect control flow effects of optional chains in equality checks. Specifically, when compiling with --strictNullChecks,

• in code guarded by o?.foo === value, where the type of value doesn't include undefined, we remove undefined and null from the type of o in the true branch, and
• in code guarded by o?.foo == value, where the type of value doesn't include undefined or null, we remove undefined and null from the type of o in the true branch, and
• in code guarded by o?.foo !== undefined, o?.foo != undefined or o?.foo != null we remove undefined and null from the type of o in the true branch, and
• in code guarded by typeof o?.foo === "xxx" or typeof o?.foo == "xxx" we remove undefined and null from the type of o in the true branch, and
• in code guarded by o?.foo instanceof xxx we remove undefined and null from the type of o in the true branch.

Some examples:

type Thing = { foo: number, bar(): number };

function foo(o: Thing | undefined, value: number) {
    if (o?.foo === value) {
        o.foo;  // Ok
    }
    if (o?.["foo"] === value) {
        o["foo"];  // Ok
    }
    if (o?.bar() === value) {
        o.bar;  // Ok
    }
    if (o?.foo !== undefined) {
        o.foo;  // Ok
    }
    if (o?.["foo"] !== undefined) {
        o["foo"];  // Ok
    }
    if (o?.bar() !== undefined) {
        o.bar;  // Ok
    }
}

Fixes #33806.

[falsandtru]

Does this support typeof o?.foo === 'number' and o?.bar instanceof Function? I couldn't find the tests for them.

[ahejlsberg]

With latest commits we only perform checks in --strictNullChecks mode. Also, latest commits add support for typeof and instanceof as well as the == and != operators.