ScheduleDAGFast.cpp is compiled to object file, but the ScheduleDAGFast
object file isn't linked into clang executable file as no symbol is
referred by outside. Add calling to createXxx of ScheduleDAGFast.cpp,
then the ScheduleDAGFast object file will be linked into clang
executable file. The static RegisterScheduler will register scheduler
fast and linearize at clang boot time.
Reviewed By: pengfei
Differential Revision: https://reviews.llvm.org/D101601