Normative: Avoid extra Zoned→Plain conversion in AddDuration

This is an addendum to part 3 (#2671) that removes a ZonedDateTime to
PlainDateTime conversion in one place where a fast path doesn't need the
PlainDateTime. This is observable in the case where you add or subtract
two Temporal.Durations that don't have any units higher than hours.

Credit to Anba for spotting this.

Closes: #2696

polyfill/lib/ecmascript.mjs

@@ -4877,7 +4877,10 @@ export function AddDuration(
     const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
     const calendar = GetSlot(zonedRelativeTo, CALENDAR);
     const startInstant = GetSlot(zonedRelativeTo, INSTANT);
-    const startDateTime = precalculatedPlainDateTime ?? GetPlainDateTimeFor(timeZoneRec, startInstant, calendar);
+    let startDateTime = precalculatedPlainDateTime;
+    if (largestUnit === 'year' || largestUnit === 'month' || largestUnit === 'week' || largestUnit === 'day') {
+      startDateTime ??= GetPlainDateTimeFor(timeZoneRec, startInstant, calendar);
+    }
     const intermediateNs = AddZonedDateTime(
       startInstant,
       timeZoneRec,

spec/duration.html

@@ -1643,7 +1643,8 @@ <h1>
           1. Return ! CreateDurationRecord(_dateDifference_.[[Years]], _dateDifference_.[[Months]], _dateDifference_.[[Weeks]], _result_.[[Days]], _result_.[[Hours]], _result_.[[Minutes]], _result_.[[Seconds]], _result_.[[Milliseconds]], _result_.[[Microseconds]], _result_.[[Nanoseconds]]).
         1. Assert: _zonedRelativeTo_ is not *undefined*.
         1. If _precalculatedPlainDateTime_ is not present, let _precalculatedPlainDateTime_ be *undefined*.
-        1. If _precalculatedPlainDateTime_ is *undefined*, then
+        1. If _largestUnit_ is *"year"*, or _largestUnit_ is *"month"*, or _largestUnit_ is *"week"*, or _largestUnit_ is *"day"*, let _startDateTimeNeeded_ be *true*; else let _startDateTimeNeeded_ be *false*.
+        1. If _precalculatedPlainDateTime_ is *undefined* and _startDateTimeNeeded_ is *true*, then
           1. Let _startDateTime_ be ? GetPlainDateTimeFor(_timeZoneRec_, _zonedRelativeTo_.[[Nanoseconds]], _calendarRec_.[[Receiver]]).
         1. Else,
           1. Let _startDateTime_ be _precalculatedPlainDateTime_.