Fix return and throw propagation for yield*

[rbuckton]

This fixes two issues in __generator:

1. We do not correctly handle iter.return() when in a yield*, as per 14.4.13 step 7.c.viii.2.
2. We do not correctly close the iterator delegated via yield* during iter.throw() when the delegated iterator does not have a throw method, as per 14.4.13 step 7.b.iii.4

This PR makes two changes to the __generator helper to fix these issues (respectively):

1. We fix iter.return() on generators.ts:3256 by preserving the "return" opcode (2).
2. We fix iter.throw() on generators.ts:3255 when y.throw is not present by calling (and ignoring the result of) y.return() if it exists.

Fixes #24288

[weswigham]

I still wish we had functional tests for these helpers, tho.

[rbuckton]

Sounds like something we should consider as part of the test262 tests?

[rbuckton]

@mhegazy do you want this ported to release-2.9 as well?

[mhegazy]

yes please.