[ScheduleDAG] Support REQ_SEQUENCE unscheduling

REG_SEQUENCE node requires special treatment during the
unscheduling because the node is untyped and neither its
class, nor cost could be retrieved the same way as for
typed nodes.

Related issue: https://github.com/llvm/llvm-project/issues/58911

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D138837