Change Projection to support signed indices.

Change ProjectionIndex for ref_tail_addr to std::numeric_limits<int>::max();
This is necessary to disambiguate the tail elements from
ref_element_addr field zero.

lib/SILOptimizer/LoopTransforms/ArrayPropertyOpt.cpp

@@ -402,7 +402,7 @@ private:
     if (!Call.canHoist(Preheader->getTerminator(), DomTree))
       return false;
 
-    SmallVector<unsigned, 4> AccessPath;
+    SmallVector<int, 4> AccessPath;
     SILValue ArrayContainer =
       StructUseCollector::getAccessPath(Arr, AccessPath);