[regalloc] Fix assertion error when LiveInterval is empty

When evicting interference, it causes an asseertion error
since LiveIntervals::intervalIsInOneMBB assumes that input
is not empty.

This patch fixed bug mentioned in D118020.

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D118124

llvm/lib/CodeGen/LiveIntervals.cpp

@@ -827,6 +827,8 @@ CancelKill:
 
 MachineBasicBlock*
 LiveIntervals::intervalIsInOneMBB(const LiveInterval &LI) const {
+  assert(!LI.empty() && "LiveInterval is empty.");
+
   // A local live range must be fully contained inside the block, meaning it is
   // defined and killed at instructions, not at block boundaries. It is not
   // live in or out of any block.

llvm/lib/CodeGen/RegAllocGreedy.cpp

@@ -497,7 +497,7 @@ bool DefaultEvictionAdvisor::canEvictInterferenceBasedOnCost(
   if (Matrix->checkInterference(VirtReg, PhysReg) > LiveRegMatrix::IK_VirtReg)
     return false;
 
-  bool IsLocal = LIS->intervalIsInOneMBB(VirtReg);
+  bool IsLocal = VirtReg.empty() || LIS->intervalIsInOneMBB(VirtReg);
 
   // Find VirtReg's cascade number. This will be unassigned if VirtReg was never
   // involved in an eviction before. If a cascade number was assigned, deny