CodeGen: Fix assertion in machine inst sheduler due to llvm.dbg.value

Two issues were found about machine inst scheduler when compiling ProRender
with -g for amdgcn target:

GCNScheduleDAGMILive::schedule tries to update LiveIntervals for DBG_VALUE, which it
should not since DBG_VALUE is not mapped in LiveIntervals.

when DBG_VALUE is the last instruction of MBB, ScheduleDAGInstrs::buildSchedGraph and
ScheduleDAGMILive::scheduleMI does not move RPTracker properly, which causes assertion.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D41132

llvm-svn: 320650