Completely eliminate VNInfo flags.

The 'unused' state of a value number can be represented as an invalid
def SlotIndex. This also exposed code that shouldn't have been looking
at unused value VNInfos.

llvm-svn: 161258
diff --git a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
index bbc8c52..b488e8f 100644
--- a/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
+++ b/llvm/lib/CodeGen/LiveIntervalAnalysis.cpp
@@ -706,7 +706,7 @@
       continue;
     if (VNI->isPHIDef()) {
       // This is a dead PHI. Remove it.
-      VNI->setIsUnused(true);
+      VNI->markUnused();
       NewLI.removeRange(*LII);
       DEBUG(dbgs() << "Dead PHI at " << VNI->def << " may separate interval\n");
       CanSeparate = true;