Fix a bug in IfConversion.cpp.

The bug is introduced in r254377 which failed some tests on ARM, where a new
probability is assigned to a successor but the provided BB may not be a
successor.

llvm-svn: 254463
diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index ff28f95..e90cb02 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -1254,9 +1254,8 @@
     auto NewNext = BBNext + BBCvt * CvtNext;
     auto NewTrueBBIter =
         std::find(BBI.BB->succ_begin(), BBI.BB->succ_end(), NewTrueBB);
-    assert(NewTrueBBIter != BBI.BB->succ_end() &&
-           "NewTrueBB is not a successor of BBI.BB.");
-    BBI.BB->setSuccProbability(NewTrueBBIter, NewNext);
+    if (NewTrueBBIter != BBI.BB->succ_end())
+      BBI.BB->setSuccProbability(NewTrueBBIter, NewNext);
 
     auto NewFalse = BBCvt * CvtFalse;
     TII->InsertBranch(*BBI.BB, CvtBBI->FalseBB, nullptr, RevCond, dl);