Editorial: Avoid GetOffsetNanosecondsFor call in InterpretISODateTime…

…Offset

Spec text algorithm corresponding to the previous commit.

spec/zoneddatetime.html

@@ -992,8 +992,9 @@ <h1>
         1. Assert: _offsetOption_ is *"prefer"* or *"reject"*.
         1. Let _possibleEpochNs_ be ? GetPossibleEpochNanoseconds(_timeZone_, _isoDateTime_).
         1. If _possibleEpochNs_ is not empty, then
+          1. Let _utcEpochNanoseconds_ be GetUTCEpochNanoseconds(_year_, _month_, _day_, _time_.[[Hour]], _time_.[[Minute]], _time_.[[Second]], _time_.[[Millisecond]], _time_.[[Microsecond]], _time_.[[Nanosecond]]).
           1. For each element _candidate_ of _possibleEpochNs_, do
-            1. Let _candidateOffset_ be GetOffsetNanosecondsFor(_timeZone_, _candidate_).
+            1. Let _candidateOffset_ be _utcEpochNanoseconds_ - _candidate_.
             1. If _candidateOffset_ = _offsetNanoseconds_, then
               1. Return _candidate_.
             1. If _matchBehaviour_ is ~match-minutes~, then