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):

• -> portable arg{max,min}: optimize update check #8755

[pytorch-bot]

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❌ 2 New Failures

As of commit c9bd251 with merge base afcec1d ():

NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
  RuntimeError: Error checking labels: PR does not have required labels
• Lint / lintrunner / linux-job (gh)
  >>> Lint for kernels/portable/cpu/op_argmin.cpp:

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