Normative: Optimize calls into user code in InterpretISODateTimeOffset
Normative: Optimize calls into user code in InterpretISODateTimeOffset

In the current spec text algorithm, there was an unnecessary call to
timeZone.getPossibleInstantsFor(), which potentially calls observably into
user code. Fix the algorithm so it doesn't make any more calls than
necessary.

js-temporal/temporal-polyfill,js-temporal/temporal-polyfill,js-temporal/temporal-polyfill