Some stackslots are allocated to vregs which have no real reference.

LiveRangeEdit::eliminateDeadDef is used to remove dead define instructions
after rematerialization. To remove a VNI for a vreg from its LiveInterval,
LiveIntervals::removeVRegDefAt is used. However, after non-PHI VNIs are all
removed, PHI VNI are still left in the LiveInterval. Such unused vregs will
be kept in RegsToSpill[] at the end of InlineSpiller::reMaterializeAll and
spiller will allocate stackslot for them.

The fix is to get rid of unused reg by checking whether it has non-dbg
reference instead of whether it has non-empty interval.

llvm-svn: 259895