[PowerPC] Duplicate inherited heuristic from base scheduler

PowerPC has its custom scheduler heuristic. It calls parent classes'
tryCandidate in override version, but the function returns void, so this
way doesn't actually help. This patch duplicates code from base scheduler
into PPC machine scheduler class, which does what we wanted.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D94464