Revert change and clean datatypes cons candidate generator (#7645)

Revert change and clean datatypes cons candidate generator (#7645)

PR #7600 refactored and optimized candidate generator. This PR actually corrected an issue where the datatypes constructor expansion candidate generator did not generate instances for triggers whose top symbol is a constructor. However, fixing it to generate instances led to a performance regression on Facebook benchmarks.

This PR reverts the behavior change from that PR, and moreover makes it explicit that we do not generate instances in this case.