[MAINT] Save repeated expression in accumulation functor operator to a variable

[oleksandr-pavlyk]

Checking in small change in accumulator header file.

It saved repeated sub-expressions into a temporary to clean up the code and hopefully reduce the size of the binary by a few bytes.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/1896/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.19.0dev0=py310hdf72452_206 ran successfully.
Passed: 895
Failed: 0
Skipped: 119

[coveralls]

coverage: 87.705%. remained the same
when pulling 1db958a on accumulator-maint-tweak
into dd2812f on master.