portable arg{max,min}: optimize update check

[swolchok]

We don't need a second isnan; see code comment. (This is a small optimization.)

[swolchok]

Stack from ghstack (oldest at bottom):

• Add optimized op_where #8866
• Deploy BroadcastIndexesRange #8865
• add BroadcastIndexesRange #8864
• -> portable arg{max,min}: optimize update check #8863

[pytorch-bot]

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❌ 1 New Failure

As of commit 3143085 with merge base 09ad20a ():

NEW FAILURE - The following job has failed:

• pull / unittest / linux / linux-job (gh)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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[swolchok]

ping @manuelcandales @kimishpatel

[swolchok]

(just rebased the branch, but tests were all green right before)