[JumpThreading] A bug fix for stale loop info after unfold select

Summary:
The return value of a TryToUnfoldSelect call was not checked, which led to an
incorrectly preserved loop info and some crash.

The original crash was reported on https://reviews.llvm.org/D59514.

Reviewers: davidxl, amehsan

Reviewed By: davidxl

Subscribers: fhahn, brzycki, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61920

llvm-svn: 360780
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index 123c8b9..7cb955d 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -1174,7 +1174,8 @@
   }
 
   if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
-    TryToUnfoldSelect(SI, BB);
+    if (TryToUnfoldSelect(SI, BB))
+      return true;
 
   // Check for some cases that are worth simplifying.  Right now we want to look
   // for loads that are used by a switch or by the condition for the branch.  If