C++: Stop caching raw IR construction predicates

These predicates are only used within the new single IR stage, so there's no need to cache them beyond that. RA diffs are trivial. Where previously many of the predicate on `Instruction` were inline wrappers around cached predicates from `IRConstruction`, now the predicates from `IRConstruction` get inlined into the `Instruction` predicates, and the `Instruction` predicates get materialized. The net amount of work is the same, but now it's not getting cached unnecessarily.

Author: Dave Bartolomeo