Fix a bug in ScheduleDAGMILive::scheduleMI which causes BotRPTracker not tracking CurrentBottom in some rare cases involving llvm.dbg.value.
This issues causes amdgcn target to assert when compiling some user codes with -g.
Differential Revision: https://reviews.llvm.org/D42394
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323214 91177308-0d34-0410-b5e6-96231b3b80d8
