Current InlineCost is not assignable because of const members Cost and Threshold.
I dont see practical benefits from having them const (access to these members is
private and internal interactions are rather simple). On other hand that makes
it hard to use as a member in some other data structure where assignability is necessary.
I'm going to use InlineCost in a downstream inliner that maintains a complex queue
of candidate call-sites and thus keeping and recalculating InlineCost is necessary.
This patch just removes 'const' from both members, making InlineCost assignable.