Use prefetch for mConstraints

[asenkew]

Hello Jorrit,
another idea is to use prefetch.
Attached patch gives positive uplifts, measurements are also attached (comparing with master, -q=Discrete -repeat=10, surely Distribution build).
I will also create PR.
patch_prefetch_mConstraints.txt
prefetch_mConstraints_measurements.txt

[jrouwe]

B.t.w. there's no need to open both a discussion and a PR. You can just open a PR that can serve as discussion too.