Reinstate constant propagation in some form
The new codegen doesn't have a constant propagation pass. This used to be done by the mini-inliner which was ad-hoc at best; instead of the mini-inliner we now have
CmmSink which is much less ad-hoc but doesn't do constant folding or propagation.
This is a regression that will affect performance of things like
copyArray#, so we should fix it before the release.