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

test/Analysis/ScalarEvolution/2012-05-18-LoopPredRecurse.ll

@@ -0,0 +1,30 @@
+; RUN: opt < %s -iv-users -S -disable-output
+;
+; PR12868: Infinite recursion:
+; getUDivExpr()->getZeroExtendExpr()->isLoopBackedgeGuardedBy()
+;
+; We actually want SCEV simplification to fail gracefully in this
+; case, so there's no output to check, just the absense of stack overflow.
+
+@c = common global i8 0, align 1
+
+define i32 @func() {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.body, %entry
+  %storemerge = phi i8 [ -1, %entry ], [ %inc, %for.body ]
+  %ui.0 = phi i32 [ undef, %entry ], [ %div, %for.body ]
+  %tobool = icmp eq i8 %storemerge, 0
+  br i1 %tobool, label %for.end, label %for.body
+
+for.body:                                         ; preds = %for.cond
+  %conv = sext i8 %storemerge to i32
+  %div = lshr i32 %conv, 1
+  %tobool2 = icmp eq i32 %div, 0
+  %inc = add i8 %storemerge, 1
+  br i1 %tobool2, label %for.cond, label %for.end
+
+for.end:                                          ; preds = %for.body, %for.cond
+  ret i32 0
+}