Simplify `RegionConstraintCollector::vars_since_snapshot`
Simplify `RegionConstraintCollector::vars_since_snapshot`

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust