[Strict FP] Allow more relaxed scheduling

Reimplement scheduling constraints for strict FP instructions in
ScheduleDAGInstrs::buildSchedGraph to allow for more relaxed
scheduling.  Specifially, allow one strict FP instruction to
be scheduled across another, as long as it is not moved across
any global barrier.

Differential Revision: https://reviews.llvm.org/D64412

Reviewed By: cameron.mcinally

llvm-svn: 366222