Allow null interference cursors to be queried.

They always report 'no interference'.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135843 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/InterferenceCache.h b/lib/CodeGen/InterferenceCache.h
index 7f0a27a..4df0a9e 100644
--- a/lib/CodeGen/InterferenceCache.h
+++ b/lib/CodeGen/InterferenceCache.h
@@ -138,6 +138,7 @@
   class Cursor {
     Entry *CacheEntry;
     BlockInterference *Current;
+    static BlockInterference NoInterference;
 
     void setEntry(Entry *E) {
       Current = 0;
@@ -175,7 +176,7 @@
 
     /// moveTo - Move cursor to basic block MBBNum.
     void moveToBlock(unsigned MBBNum) {
-      Current = CacheEntry->get(MBBNum);
+      Current = CacheEntry ? CacheEntry->get(MBBNum) : &NoInterference;
     }
 
     /// hasInterference - Return true if the current block has any interference.