split cdd_u64 into two functions to avoid long optimization times

[rainers]

This function takes more than 10 seconds on my system in the optimizer, but was the only function that takes more than 2 seconds.

Splitting it into two functions moves them somewhere into the 1-2 second interval. There are a about a dozen other functions that need similar time to optimize.

This is trying to recover reproducible builds, see #10238 (comment)

[dlang-bot]

Thanks for your pull request, @rainers!

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dub fetch digger
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