[NFC][Scheduler] Refactor tryCandidate to return boolean

This patch changes return type of tryCandidate from void to bool:

1. Methods in some targets already follow this convention.
2. This would help if some target wants to re-use generic code.
3. It looks more intuitive if these try-method returns the same type.

We may need to change return type of them from bool to some enum
further, to make it less confusing.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D103951