SCEV: Add MarkPendingLoopPredicates to avoid recursive isImpliedCond.

getUDivExpr attempts to simplify by checking for overflow.
isLoopEntryGuardedByCond then evaluates the loop predicate which
may lead to the same getUDivExpr causing endless recursion.

Fixes PR12868: clang 3.2 segmentation fault.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157092 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolution.h

@@ -30,7 +30,7 @@
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/ConstantRange.h"
 #include "llvm/ADT/FoldingSet.h"
-#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DenseSet.h"
 #include <map>
 
 namespace llvm {
@@ -250,6 +250,9 @@ namespace llvm {
     ///
     ValueExprMapType ValueExprMap;
 
+    /// Mark predicate values currently being processed by isImpliedCond.
+    DenseSet<Value*> PendingLoopPredicates;
+
     /// ExitLimit - Information about the number of loop iterations for
     /// which a loop exit's branch condition evaluates to the not-taken path.
     /// This is a temporary pair of exact and max expressions that are