incr.comp.: Optimize DepGraph::try_mark_green()

[michaelwoerister]

DepGraph::try_mark_green() is a rather hot function, especially in compilation sessions that see a lot of re-use. It can probably be made faster by:

• using an array instead of a hashmap for storing node colors,
• and maybe, by re-using the current_deps vector among recursive invocations or using a SmallVec (although such optimizations can easily backfire, so proper profiling is needed here).

[michaelwoerister]

#48206 implements the first bullet point.

[gizmondo]

Should be closed per #50565?

[wesleywiser]

Indeed, thanks @gizmondo!