DenseMap: assert that we have found a bucket before we try to insert into it.

This silences literally dozens of analyzer warnings on LLVM (since DenseMap
is such a commonly-used class).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164438 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/DenseMap.h

@@ -423,6 +423,7 @@ private:
       this->grow(NumBuckets);
       LookupBucketFor(Key, TheBucket);
     }
+    assert(TheBucket);
 
     // Only update the state after we've grown our bucket space appropriately
     // so that when growing buckets we have self-consistent entry count.