(perf) optimize assign-depths

[dmichon-msft]

Roughly 5% of my team's watch-mode rebuilds is spent in the assignDepths function. This change reduces that to 3%

What kind of change does this PR introduce?
performance enhancement

Did you add tests for your changes?
Covered by existing unit tests for this function. Not a logic change.

Does this PR introduce a breaking change?
No.

What needs to be documented once your changes are merged?
Performance only, no behavior change, so nothing.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[alexander-akait]

Thanks